distribution: fallbackError, notFoundError implement go1.13 unwrapper

These errors implemented the Causer interface, but did not implement
the go1.13 unwrapper, which could prevent errors from being matched.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

distribution/errors.go

@@ -32,13 +32,17 @@ type fallbackError struct {
 
 // Error renders the FallbackError as a string.
 func (f fallbackError) Error() string {
-	return f.Cause().Error()
+	return f.err.Error()
 }
 
 func (f fallbackError) Cause() error {
 	return f.err
 }
 
+func (f fallbackError) Unwrap() error {
+	return f.err
+}
+
 type notFoundError struct {
 	cause errcode.Error
 	ref   reference.Named
@@ -64,6 +68,10 @@ func (e notFoundError) Cause() error {
 	return e.cause
 }
 
+func (e notFoundError) Unwrap() error {
+	return e.cause
+}
+
 // unsupportedMediaTypeError is an error issued when attempted
 // to pull unsupported content.
 type unsupportedMediaTypeError struct {