Update xbilling.php

this file had the problem that lost error message for example "Please provide a valid username." in the line 135:

        $res = $xmws->XMLDataToArray($xmws->Request($xmws->BuildRequest()));

for this reasen, i capture this message in a new varible to show if response is diferent that 1101

        if($post_parms){
            $xmws->SetRequestData($post_parms);
            $answer = $xmws->Request($xmws->BuildRequest());//Please provide a valid username.
            $res = $xmws->XMLDataToArray($answer);
        }
        if ($res['xmws']['response'] != '1101') {
            die("API Error: " . $res['xmws']['content']. $answer);
        }