func IsErrCredentialsNotFound(err error) bool {
	_, ok := err.(errCredentialsNotFound)
	return ok
}
IsErrCredentialsNotFound returns true if the error
was caused by not having a set of credentials in a store.
package credentials
const (
	errCredentialsNotFoundMessage = "credentials not found in native keychain"
	errCredentialsMissingServerURLMessage = "no credentials server URL"
	errCredentialsMissingUsernameMessage  = "no credentials username"
)
type errCredentialsNotFound struct{}
func (errCredentialsNotFound) Error() string {
	return errCredentialsNotFoundMessage
}
func NewErrCredentialsNotFound() error {
	return errCredentialsNotFound{}
}