fix: update error in SignedEntity to be more descriptive

cmd/cosign/cli/sign/sign.go

@@ -179,7 +179,7 @@ func SignCmd(ro *options.RootOptions, ko options.KeyOpts, signOpts options.SignO
 
 		if digest, ok := ref.(name.Digest); ok && !signOpts.Recursive {
 			se, err := ociremote.SignedEntity(ref, opts...)
-			if err == ociremote.ErrEntityNotFound {
+			if _, isEntityNotFoundErr := err.(*ociremote.EntityNotFoundError); isEntityNotFoundErr {
 				se = ociremote.SignedUnknown(digest)
 			} else if err != nil {
 				return fmt.Errorf("accessing image: %w", err)

pkg/oci/remote/remote.go

@@ -36,12 +36,24 @@ var (
 	remoteIndex = remote.Index
 	remoteGet   = remote.Get
 	remoteWrite = remote.Write
-
-	// ErrEntityNotFound is the error that SignedEntity returns when the
-	// provided ref does not exist.
-	ErrEntityNotFound = errors.New("entity not found in registry")
 )
 
+// EntityNotFoundError is the error that SignedEntity returns when the
+// provided ref does not exist.
+type EntityNotFoundError struct {
+	baseErr error
+}
+
+func (e *EntityNotFoundError) Error() string {
+	return fmt.Sprintf("entity not found in registry, error: %v", e.baseErr)
+}
+
+func NewEntityNotFoundError(err error) error {
+	return &EntityNotFoundError{
+		baseErr: err,
+	}
+}
+
 // SignedEntity provides access to a remote reference, and its signatures.
 // The SignedEntity will be one of SignedImage or SignedImageIndex.
 func SignedEntity(ref name.Reference, options ...Option) (oci.SignedEntity, error) {
@@ -50,7 +62,7 @@ func SignedEntity(ref name.Reference, options ...Option) (oci.SignedEntity, erro
 	got, err := remoteGet(ref, o.ROpt...)
 	var te *transport.Error
 	if errors.As(err, &te) && te.StatusCode == http.StatusNotFound {
-		return nil, ErrEntityNotFound
+		return nil, NewEntityNotFoundError(err)
 	} else if err != nil {
 		return nil, err
 	}