发送网络请求有时失败，分析一下响应行，在响应的响应行内，你会发现响应行由三部分组成，用空格来隔开，HTTP/1.1 404 NOT FOUND，第一个是响应的HTTP的版本，第二个和第三个是状态值。
发送完网络请求会得到一个Error和response对象，我们不能单单只判断Error，直接打印出error这个错误，我们应该分析respon的属性statusCode，然后根据状态码分析错误的原因
分析状态值来分析网络请求错误的原因
分为五类错误
下面列出了具体的网络请求错误
|"100" ; Section 10.1.1: Continue | "101" ; Section 10.1.2: Switching Protocols | "200" ; Section 10.2.1: OK | "201" ; Section 10.2.2: Created | "202" ; Section 10.2.3: Accepted | "203" ; Section 10.2.4: Non-Authoritative Information | "204" ; Section 10.2.5: No Content | "205" ; Section 10.2.6: Reset Content | "206" ; Section 10.2.7: Partial Content | "300" ; Section 10.3.1: Multiple Choices | "301" ; Section 10.3.2: Moved Permanently | "302" ; Section 10.3.3: Found | "303" ; Section 10.3.4: See Other | "304" ; Section 10.3.5: Not Modified | "305" ; Section 10.3.6: Use Proxy | "307" ; Section 10.3.8: Temporary Redirect | "400" ; Section 10.4.1: Bad Request | "401" ; Section 10.4.2: Unauthorized | "402" ; Section 10.4.3: Payment Required | "403" ; Section 10.4.4: Forbidden | "404" ; Section 10.4.5: Not Found | "405" ; Section 10.4.6: Method Not Allowed | "406" ; Section 10.4.7: Not Acceptable
| "407" ; Section 10.4.8: Proxy Authentication Required
| "408" ; Section 10.4.9: Request Time-out
| "409" ; Section 10.4.10: Conflict
| "410" ; Section 10.4.11: Gone
| "411" ; Section 10.4.12: Length Required
| "412" ; Section 10.4.13: Precondition Failed
| "413" ; Section 10.4.14: Request Entity Too Large
| "414" ; Section 10.4.15: Request-URI Too Large
| "415" ; Section 10.4.16: Unsupported Media Type
| "416" ; Section 10.4.17: Requested range not satisfiable
| "417" ; Section 10.4.18: Expectation Failed
| "500" ; Section 10.5.1: Internal Server Error
| "501" ; Section 10.5.2: Not Implemented
| "502" ; Section 10.5.3: Bad Gateway
| "503" ; Section 10.5.4: Service Unavailable
| "504" ; Section 10.5.5: Gateway Time-out
| "505" ; Section 10.5.6: HTTP Version not supported