请教一个ASP.NET+AJAX异常处理的问题！

我的ASP.NET中使用了Ajax ，现在遇到一个麻烦，在程序遇到错误时无任何反应！
没试的代码：
<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="index.aspx.cs" Inherits="aspnet.index" %>
<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
    <asp:ScriptManager ID="ScriptManager1" runat="server" 
        onasyncpostbackerror="ScriptManager1_AsyncPostBackError">
    </asp:ScriptManager>
    <div>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
                /<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
                =<asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
                <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="计算" />
            </ContentTemplate>
            <Triggers>
                <asp:AsyncPostBackTrigger ControlID="Button1" EventName="Click" />
            </Triggers>
        </asp:UpdatePanel>
    </div>
</asp:Content>cs:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;namespace aspnet
{
    public partial class index : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {        }        protected void Button1_Click(object sender, EventArgs e)
        {
            try
            {
                int a = Int32.Parse(TextBox1.Text);
                int b = Int32.Parse(TextBox2.Text);
                int res = a / b;
                Label1.Text = res.ToString();
            }
            catch(Exception ex)
            {
                if (TextBox1.Text.Length > 0 && TextBox2.Text.Length > 0)
                {
                    ex.Data["ExtraInfo"] = " You can't divide " +
                      TextBox1.Text + " by " + TextBox2.Text;
                    throw ex;
                }
            }
        }        protected void ScriptManager1_AsyncPostBackError(object sender, AsyncPostBackErrorEventArgs e)
        {
            if (e.Exception.Data["ExtraInfo"] != null)
            {
                ScriptManager1.AsyncPostBackErrorMessage =
                   e.Exception.Message +
                   e.Exception.Data["ExtraInfo"].ToString();
            }
            else
            {
                ScriptManager1.AsyncPostBackErrorMessage = "An unspecified error occurred.";
            }
        }
    }
}