Add validation error to digesting reader

The reason is hidden if `expectedDigest.Validate()` errors. This patch
will add the error message to the stack to provide more details.

Signed-off-by: Sascha Grunert <[email protected]>

copy/digesting_reader.go

@@ -23,7 +23,7 @@ type digestingReader struct {
 func newDigestingReader(source io.Reader, expectedDigest digest.Digest) (*digestingReader, error) {
 	var digester digest.Digester
 	if err := expectedDigest.Validate(); err != nil {
-		return nil, fmt.Errorf("Invalid digest specification %s", expectedDigest)
+		return nil, fmt.Errorf("Invalid digest specification %s: %w", expectedDigest, err)
 	}
 	digestAlgorithm := expectedDigest.Algorithm()
 	if !digestAlgorithm.Available() {