distribution: Error when pulling OCI artifacts
Currently an attempt to pull a reference which resolves to an OCI artifact (Helm chart for example), results in a bit unrelated error message `invalid rootfs in image configuration`.

**- What I did**
This provides a more meaningful error in case a user attempts to download a media type which isn't image related.

**- How to verify it**
`docker pull docker.io/docker/demo:0.1.0`
Should return: `unsupported media type application/vnd.cncf.helm.config.v1+json`

**- Description for the changelog**
<!--
Write a short (one line) summary that describes the changes in this
pull request for inclusion in the changelog:
-->

**- A picture of a cute animal (not mandatory but encouraged)**
Are there some other media types which we need to support?


distribution/errors.go
package distribution // import "github.com/docker/docker/distribution"

import (
	"fmt"
	"net/url"
	"strings"
	"syscall"

	"github.com/docker/distribution"
	"github.com/docker/distribution/reference"
	"github.com/docker/distribution/registry/api/errcode"
	v2 "github.com/docker/distribution/registry/api/v2"
	"github.com/docker/distribution/registry/client"
	"github.com/docker/distribution/registry/client/auth"
	"github.com/docker/docker/distribution/xfer"
	"github.com/docker/docker/errdefs"
	"github.com/pkg/errors"
	"github.com/sirupsen/logrus"
)

// fallbackError wraps an error that can possibly allow fallback to a different
// endpoint.
type fallbackError struct {
	// err is the error being wrapped.
	err error
	// transportOK is set to true if we managed to speak HTTP with the
	// registry. This confirms that we're using appropriate TLS settings
	// (or lack of TLS).
	transportOK bool
}

// Error renders the FallbackError as a string.
func (f fallbackError) Error() string {
	return f.Cause().Error()
}

func (f fallbackError) Cause() error {
	return f.err
}

type notFoundError struct {
	cause errcode.Error
	ref   reference.Named
}

func (e notFoundError) Error() string {
	switch e.cause.Code {
	case errcode.ErrorCodeDenied:
		// ErrorCodeDenied is used when access to the repository was denied
		return errors.Wrapf(e.cause, "pull access denied for %s, repository does not exist or may require 'docker login'", reference.FamiliarName(e.ref)).Error()
	case v2.ErrorCodeManifestUnknown:
		return errors.Wrapf(e.cause, "manifest for %s not found", reference.FamiliarString(e.ref)).Error()
	case v2.ErrorCodeNameUnknown:
		return errors.Wrapf(e.cause, "repository %s not found", reference.FamiliarName(e.ref)).Error()
	}
	// Shouldn't get here, but this is better than returning an empty string
	return e.cause.Message
}

func (e notFoundError) NotFound() {}

func (e notFoundError) Cause() error {
	return e.cause
}

// translatePullError is used to convert an error from a registry pull
// operation to an error representing the entire pull operation. Any error
// information which is not used by the returned error gets output to
// log at info level.
func translatePullError(err error, ref reference.Named) error {
	switch v := err.(type) {
	case errcode.Errors:
		if len(v) != 0 {
			for _, extra := range v[1:] {
				logrus.WithError(extra).Infof("Ignoring extra error returned from registry")
			}
			return translatePullError(v[0], ref)
		}
	case errcode.Error:
		switch v.Code {
		case errcode.ErrorCodeDenied, v2.ErrorCodeManifestUnknown, v2.ErrorCodeNameUnknown:
			return notFoundError{v, ref}
		}
	case xfer.DoNotRetry:
		return translatePullError(v.Err, ref)
	}

	return errdefs.Unknown(err)
}

func isNotFound(err error) bool {
	switch v := err.(type) {
	case errcode.Errors:
		for _, e := range v {
			if isNotFound(e) {
				return true
			}
		}
	case errcode.Error:
		switch v.Code {
		case errcode.ErrorCodeDenied, v2.ErrorCodeManifestUnknown, v2.ErrorCodeNameUnknown:
			return true
		}
	}
	return false
}

// continueOnError returns true if we should fallback to the next endpoint
// as a result of this error.
func continueOnError(err error, mirrorEndpoint bool) bool {
	switch v := err.(type) {
	case errcode.Errors:
		if len(v) == 0 {
			return true
		}
		return continueOnError(v[0], mirrorEndpoint)
	case errcode.Error:
		return mirrorEndpoint
	case *client.UnexpectedHTTPResponseError:
		return true
	case imageConfigPullError:
		// imageConfigPullError only happens with v2 images, v1 fallback is
		// unnecessary.
		// Failures from a mirror endpoint should result in fallback to the
		// canonical repo.
		return mirrorEndpoint
	case error:
		return !strings.Contains(err.Error(), strings.ToLower(syscall.ESRCH.Error()))
	}
	// let's be nice and fallback if the error is a completely
	// unexpected one.
	// If new errors have to be handled in some way, please
	// add them to the switch above.
	return true
}

// retryOnError wraps the error in xfer.DoNotRetry if we should not retry the
// operation after this error.
func retryOnError(err error) error {
	switch v := err.(type) {
	case errcode.Errors:
		if len(v) != 0 {
			return retryOnError(v[0])
		}
	case errcode.Error:
		switch v.Code {
		case errcode.ErrorCodeUnauthorized, errcode.ErrorCodeUnsupported, errcode.ErrorCodeDenied, errcode.ErrorCodeTooManyRequests, v2.ErrorCodeNameUnknown:
			return xfer.DoNotRetry{Err: err}
		}
	case *url.Error:
		switch v.Err {
		case auth.ErrNoBasicAuthCredentials, auth.ErrNoToken:
			return xfer.DoNotRetry{Err: v.Err}
		}
		return retryOnError(v.Err)
	case *client.UnexpectedHTTPResponseError:
		return xfer.DoNotRetry{Err: err}
	case error:
		if err == distribution.ErrBlobUnknown {
			return xfer.DoNotRetry{Err: err}
		}
		if strings.Contains(err.Error(), strings.ToLower(syscall.ENOSPC.Error())) {
			return xfer.DoNotRetry{Err: err}
		}
	}
	// let's be nice and fallback if the error is a completely
	// unexpected one.
	// If new errors have to be handled in some way, please
	// add them to the switch above.
	return err
}

type invalidManifestClassError struct {
	mediaType string
	class     string
}

func (e invalidManifestClassError) Error() string {
	return fmt.Sprintf("Encountered remote %q(%s) when fetching", e.mediaType, e.class)
}

func (e invalidManifestClassError) InvalidParameter() {}

type invalidManifestFormatError struct{}

func (invalidManifestFormatError) Error() string {
	return "unsupported manifest format"
}

func (invalidManifestFormatError) InvalidParameter() {}

type reservedNameError string

func (e reservedNameError) Error() string {
	return "'" + string(e) + "' is a reserved name"
}

func (e reservedNameError) Forbidden() {}

package distribution // import "github.com/docker/docker/distribution"

import (
	"fmt"
	"net/url"
	"strings"
	"syscall"

	"github.com/docker/distribution"
	"github.com/docker/distribution/reference"
	"github.com/docker/distribution/registry/api/errcode"
	v2 "github.com/docker/distribution/registry/api/v2"
	"github.com/docker/distribution/registry/client"
	"github.com/docker/distribution/registry/client/auth"
	"github.com/docker/docker/distribution/xfer"
	"github.com/docker/docker/errdefs"
	"github.com/pkg/errors"
	"github.com/sirupsen/logrus"
)

// fallbackError wraps an error that can possibly allow fallback to a different
// endpoint.
type fallbackError struct {
	// err is the error being wrapped.
	err error
	// transportOK is set to true if we managed to speak HTTP with the
	// registry. This confirms that we're using appropriate TLS settings
	// (or lack of TLS).
	transportOK bool
}

// Error renders the FallbackError as a string.
func (f fallbackError) Error() string {
	return f.Cause().Error()
}

func (f fallbackError) Cause() error {
	return f.err
}

type notFoundError struct {
	cause errcode.Error
	ref   reference.Named
}

func (e notFoundError) Error() string {
	switch e.cause.Code {
	case errcode.ErrorCodeDenied:
		// ErrorCodeDenied is used when access to the repository was denied
		return errors.Wrapf(e.cause, "pull access denied for %s, repository does not exist or may require 'docker login'", reference.FamiliarName(e.ref)).Error()
	case v2.ErrorCodeManifestUnknown:
		return errors.Wrapf(e.cause, "manifest for %s not found", reference.FamiliarString(e.ref)).Error()
	case v2.ErrorCodeNameUnknown:
		return errors.Wrapf(e.cause, "repository %s not found", reference.FamiliarName(e.ref)).Error()
	}
	// Shouldn't get here, but this is better than returning an empty string
	return e.cause.Message
}

func (e notFoundError) NotFound() {}

func (e notFoundError) Cause() error {
	return e.cause
}

// unsupportedMediaTypeError is an error issued when attempted
// to pull unsupported content.
type unsupportedMediaTypeError struct {
	MediaType string
}

func (e unsupportedMediaTypeError) InvalidParameter() {}

// Error returns the error string for unsupportedMediaTypeError.
func (e unsupportedMediaTypeError) Error() string {
	return "unsupported media type " + e.MediaType
}

// translatePullError is used to convert an error from a registry pull
// operation to an error representing the entire pull operation. Any error
// information which is not used by the returned error gets output to
// log at info level.
func translatePullError(err error, ref reference.Named) error {
	switch v := err.(type) {
	case errcode.Errors:
		if len(v) != 0 {
			for _, extra := range v[1:] {
				logrus.WithError(extra).Infof("Ignoring extra error returned from registry")
			}
			return translatePullError(v[0], ref)
		}
	case errcode.Error:
		switch v.Code {
		case errcode.ErrorCodeDenied, v2.ErrorCodeManifestUnknown, v2.ErrorCodeNameUnknown:
			return notFoundError{v, ref}
		}
	case xfer.DoNotRetry:
		return translatePullError(v.Err, ref)
	}

	return errdefs.Unknown(err)
}

func isNotFound(err error) bool {
	switch v := err.(type) {
	case errcode.Errors:
		for _, e := range v {
			if isNotFound(e) {
				return true
			}
		}
	case errcode.Error:
		switch v.Code {
		case errcode.ErrorCodeDenied, v2.ErrorCodeManifestUnknown, v2.ErrorCodeNameUnknown:
			return true
		}
	}
	return false
}

// continueOnError returns true if we should fallback to the next endpoint
// as a result of this error.
func continueOnError(err error, mirrorEndpoint bool) bool {
	switch v := err.(type) {
	case errcode.Errors:
		if len(v) == 0 {
			return true
		}
		return continueOnError(v[0], mirrorEndpoint)
	case errcode.Error:
		return mirrorEndpoint
	case *client.UnexpectedHTTPResponseError:
		return true
	case imageConfigPullError:
		// imageConfigPullError only happens with v2 images, v1 fallback is
		// unnecessary.
		// Failures from a mirror endpoint should result in fallback to the
		// canonical repo.
		return mirrorEndpoint
	case unsupportedMediaTypeError:
		return false
	case error:
		return !strings.Contains(err.Error(), strings.ToLower(syscall.ESRCH.Error()))
	}
	// let's be nice and fallback if the error is a completely
	// unexpected one.
	// If new errors have to be handled in some way, please
	// add them to the switch above.
	return true
}

// retryOnError wraps the error in xfer.DoNotRetry if we should not retry the
// operation after this error.
func retryOnError(err error) error {
	switch v := err.(type) {
	case errcode.Errors:
		if len(v) != 0 {
			return retryOnError(v[0])
		}
	case errcode.Error:
		switch v.Code {
		case errcode.ErrorCodeUnauthorized, errcode.ErrorCodeUnsupported, errcode.ErrorCodeDenied, errcode.ErrorCodeTooManyRequests, v2.ErrorCodeNameUnknown:
			return xfer.DoNotRetry{Err: err}
		}
	case *url.Error:
		switch v.Err {
		case auth.ErrNoBasicAuthCredentials, auth.ErrNoToken:
			return xfer.DoNotRetry{Err: v.Err}
		}
		return retryOnError(v.Err)
	case *client.UnexpectedHTTPResponseError, unsupportedMediaTypeError:
		return xfer.DoNotRetry{Err: err}
	case error:
		if err == distribution.ErrBlobUnknown {
			return xfer.DoNotRetry{Err: err}
		}
		if strings.Contains(err.Error(), strings.ToLower(syscall.ENOSPC.Error())) {
			return xfer.DoNotRetry{Err: err}
		}
	}
	// let's be nice and fallback if the error is a completely
	// unexpected one.
	// If new errors have to be handled in some way, please
	// add them to the switch above.
	return err
}

type invalidManifestClassError struct {
	mediaType string
	class     string
}

func (e invalidManifestClassError) Error() string {
	return fmt.Sprintf("Encountered remote %q(%s) when fetching", e.mediaType, e.class)
}

func (e invalidManifestClassError) InvalidParameter() {}

type invalidManifestFormatError struct{}

func (invalidManifestFormatError) Error() string {
	return "unsupported manifest format"
}

func (invalidManifestFormatError) InvalidParameter() {}

type reservedNameError string

func (e reservedNameError) Error() string {
	return "'" + string(e) + "' is a reserved name"
}

func (e reservedNameError) Forbidden() {}

Does `retryOnError()` also need a check for this error? https://github.com/moby/moby/blob/9184f0b5e40cd94fdb96d8fe42f74f028aa17b9a/distribution/errors.go#L139