When you use regular ASP.NET page postbacks, an error message is displayed on the page whenever an exception is thrown on the server. When you use asynchronous postbacks and UpdatePanels, the error message isn't displayed on the full page but is instead displayed in a JavaScript alert box, as shown in Figure 17.

Figure 17. A server-side exception as displayed by default

The error message that gets displayed client-side in case an exception is thrown can be modified from the server by modifying the AsyncPostBackErrorMessage property on the ScriptManager control. This is best done from the AsyncPostBackError event of the ScriptManager control. Here's how you can add a handler for the error event:

<asp:ScriptManager runat="server" ID="SM1" OnAsyncPostBackError="AsyncError" />

And here's an example of an error handler that modifies the message that will get displayed on the client when an exception is thrown from the server:

protected void AsyncError(object sender, AsyncPostBackErrorEventArgs e) {
    ScriptManager.GetCurrent(this).AsyncPostBackErrorMessage =
        "Something terribly wrong happened: " +
        e.Exception.Message;
}