added errors for every request to RollongCurl

It is not documented but sometimes curl_getinfo returns 0.
According to user comments this means that the host was not found, according to my test it seems to be true, so I added support for that.

Also sometimes an individual request would be downloaded halfway but still appear as if it was successfully downloaded, this can be checked by comparing the value in $done['result'], documentation on PHP http://www.php.net//manual/en/curl.constants.php and http://curl.haxx.se/libcurl/c/libcurl-errors.html older versions of PHP do not support curl_strerror so I added curl_multi_strerror instead, the user is free to check for the error using curl_strerror in the callback