Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
 function EndRequestHandler(sender, args){
         if (args.get_error() != undefined && args.get_error().httpStatusCode == '500')
         {
            args.set_errorHandled(true);var errorMessage = args.get_error().message;
            errorMessage=errorMessage.replace(/Sys.WebForms.PageRequestManagerServerErrorException:/,"");
            alert(errorMessage);
         }
 }