func Backoff(msg string) Backoffer {
	return BackoffError{
		message:       msg,
		shouldBackoff: true,
	}
}
Backoff returns an error that implements the Backoffer interface, telling the caller
processing the error to slow the queue down.
package pgq
type Backoffer interface {
	error
	Backoff() bool
}
type BackoffError struct {
	message       string
	shouldBackoff bool
}
func (b BackoffError) Error() string {
	return b.message
}
func (b BackoffError) Backoff() bool {
	return b.shouldBackoff
}