CURL 开发笔记
常见error no，保存在curl.h
CURLE_COULDNT_CONNECT, /* 7 */
CURLE_OPERATION_TIMEOUTED, /* 28 - the timeout time was reached */
CURLE_GOT_NOTHING, /* 52 - when this is a specific error */

reference
http://www.cnblogs.com/xwblog/archive/2011/11/09/2243080.html