Update: added ErrorPushSignatureFailed (#271)

This PR adds a `ErrorPushSignatureFailed` error type so that users of notation.Sign() would be able to check error type on Signing failures. On notation CLI side, if the error is of type `ErrorPushSignatureFailed`, a suggestion of using OCI image manifest will be printed out.

Signed-off-by: Patrick Zheng <patrickzheng@microsoft.com>

errors.go

@@ -1,5 +1,17 @@
 package notation
 
+//
+type ErrorPushSignatureFailed struct {
+	Msg string
+}
+
+func (e ErrorPushSignatureFailed) Error() string {
+	if e.Msg != "" {
+		return "failed to push signature to registry with error: " + e.Msg
+	}
+	return "failed to push signature to registry"
+}
+
 // ErrorVerificationInconclusive is used when signature verification fails due to a runtime error (e.g. a network error)
 type ErrorVerificationInconclusive struct {
 	Msg string

notation.go

@@ -111,7 +111,7 @@ func Sign(ctx context.Context, signer Signer, repo registry.Repository, opts Sig
 	_, _, err = repo.PushSignature(ctx, opts.SignatureMediaType, sig, targetDesc, annotations)
 	if err != nil {
 		logger.Error("Failed to push the signature")
-		return ocispec.Descriptor{}, err
+		return ocispec.Descriptor{}, ErrorPushSignatureFailed{Msg: err.Error()}
 	}
 
 	return targetDesc, nil
@@ -339,4 +339,4 @@ func generateAnnotations(signerInfo *signature.SignerInfo, annotations map[strin
 
 	annotations[annotationX509ChainThumbprint] = string(val)
 	return annotations, nil
-}
+}