https://zh.wikipedia.org/wiki/HTTP%E7%8A%B6%E6%80%81%E7%A0%81#5xx%E6%9C%8D%E5%8A%A1%E5%99%A8%E9%94%99%E8%AF%AF
https://segmentfault.com/q/1010000006656545
http状态码的含义及502, 503和504的区别
原文：https://www.cnblogs.com/shengulong/p/10206678.html
