Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);

//
// Displays the error message, as defined in 'ScriptManager.AsyncPostBackErrorMessage'.
//
function EndRequestHandler(sender, args) {
	var error = args.get_error();
	if (error != undefined) {
		alert(error.message.replace(error.name + ': ', ''));
		args.set_errorHandled(true);
	}
}