var prm = Sys.WebForms.PageRequestManager.getInstance();
prm.add_endRequest(EndRequest);

function EndRequest(sender, args)
{
	if (args.get_error())
	{
		var errorMessage = args.get_error().message;
		errorMessage = errorMessage.replace("Sys.WebForms.PageRequestManagerServerErrorException: ", "");
		alert(errorMessage);
		args.set_errorHandled(true);
	}
}