ErrorCode

English
1000	download file already exists,and {@link DownloadConfig#forceReDownload} is false
1001	Network unavailable
1002	Website return 404
1003	Server unknown error
1004	Breakpoint download need request file's length first,but not found in http response head,connect server to resolve it

中文
1000	文件已经下载好了,并且 DownloadConfig#forceReDownload设置的false
1001	网络不可用
1002	网站返回404
1003	未知服务器异常
1004	获取不到下载文件大小