Description
The last error message (if any) returned from a SCRIPT call
取得最后的错误消息。
Parameters
none
Return Value
A string with the last returned script based error message, or NULL if there is no error
如果有错误，返回一个字符串；如果没有错误，返回NULL。
Examples
$redis->eval('this-is-not-lua');
$err = $redis->getLastError();
// "ERR Error compiling script (new function): user_script:1: '=' expected near '-'"
