$(document).ready(function() {
	Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endRequestHandler);
});

function endRequestHandler(sender, eventArgs) {
	if (eventArgs.get_error() != undefined &&
        eventArgs.get_error().httpStatusCode == '500') {
		var errorMessage = eventArgs.get_error().toString();
		eventArgs.set_errorHandled(true);
		alert(errorMessage);
	}
}