file: Fix error message for DefaultBucketName failures (#201)

file/file.go

@@ -22,7 +22,7 @@ func DefaultBucketName(c context.Context) (string, error) {
 
 	err := internal.Call(c, "app_identity_service", "GetDefaultGcsBucketName", req, res)
 	if err != nil {
-		return "", fmt.Errorf("file: no default bucket name returned in RPC response: %v", res)
+		return "", fmt.Errorf("file: no default bucket name returned in RPC response: %v", err)
 	}
 	return res.GetDefaultGcsBucketName(), nil
 }