function _onErrorMergeResult(res){
            _showMergeResult({
                "message" : Messages("pullRequest.error.newPullRequestForm", res.status, res.statusText),
                "cssClass": "alert-error"
            });
            _fillFormTitleBody({"title":"", "body":""});
        }
On error occurs to get merge result.
Show error message and response status.

@param res
@private