当swift中服务器端发生异常时，无法获得http响应

I am using NSURLSession to request http post to server. It works fine if everything is ok. But I can't get the response when the server throw an exception. In browser, I should be able to get the 505 error code. But I can't get it in swift. Below is the code I used. Is there anything wrong with it?I am using NSURLSession to request http post to