验证表单怎么返回错误

我看了看request的validate方法，验证失败直接就抛出一个异常然后渲染到模板，但是如果我做API的话，需要返回指定格式的JSON，应该怎么在控制器里获取到这个错误呢？
比如

    public function store(Request $req) {
        $this->validate($req,['title'=>'required','content'=>'required']);

        //if doesn't pass the validation how can I get the error info and return json such as ['msg'=>$errorinfo]

       //if passed
       $article = App\Article::create($req->all());
       return ['msg'=>'published success','article'=>$article];
    }