add string desc for socket error about fastdfs HOT 5 CLOSED

The error code is unix error number.
For detail info, you can see the header file in the unix system: 
/usr/include/errno.h
In Linux, the destination file is: /usr/include/asm/errno.h

for example:
28 means no space left on device, the storage server is full

#define ENOSPC          28      /* No space left on device */

in php, thereis a function socket_strerror，which can return the string desc 
of a 
error number. 

the error no 28 is not the socket errno, it's unix errno.
so you should see the c header file: /usr/include/errno.h