image: Return an error message from fit_config_verify_sig()

This function only returns an error message sometimes. Update it to always
return an error message if one is available. This makes it easier to see
what went wrong.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/image-sig.c b/common/image-sig.c
index 639a112..13ccd50 100644
--- a/common/image-sig.c
+++ b/common/image-sig.c
@@ -499,13 +499,14 @@
 		goto error;
 	}
 
-	return verified ? 0 : -EPERM;
+	if (verified)
+		return 0;
 
 error:
 	printf(" error!\n%s for '%s' hash node in '%s' config node\n",
 	       err_msg, fit_get_name(fit, noffset, NULL),
 	       fit_get_name(fit, conf_noffset, NULL));
-	return -1;
+	return -EPERM;
 }
 
 int fit_config_verify_required_sigs(const void *fit, int conf_noffset,