verify-blob: make the signature flag mandatory (#1045)

Before this commit, the verify-blob command would fail ungracefully in experimental mode
if no signature has been provided. This commit adds a better fitting error message.

Signed-off-by: Christian Rebischke <chris@shibumi.dev>

cmd/cosign/cli/verify/verify_blob.go

@@ -62,6 +62,9 @@ func VerifyBlobCmd(ctx context.Context, ko sign.KeyOpts, certRef, sigRef, blobRe
 	}
 
 	var b64sig string
+	if sigRef == "" {
+		return fmt.Errorf("missing flag '--signature'")
+	}
 	targetSig, err := blob.LoadFileOrURL(sigRef)
 	if err != nil {
 		if !os.IsNotExist(err) {