Add a generic error type to capture non-typed errors

Signed-off-by: Richard Scothern <richard.scothern@gmail.com>

registry/storage/driver/inmemory/driver.go

@@ -258,5 +258,5 @@ func (d *driver) Delete(ctx context.Context, path string) error {
 // URLFor returns a URL which may be used to retrieve the content stored at the given path.
 // May return an UnsupportedMethodErr in certain StorageDriver implementations.
 func (d *driver) URLFor(ctx context.Context, path string, options map[string]interface{}) (string, error) {
-	return "", new(storagedriver.ErrUnsupportedMethod)
+	return "", storagedriver.ErrUnsupportedMethod{}
 }