Browser Not Handling Exception From AJAX Panel - C#

i am having trouble catching errors in an AJAX panel. Even when i throw an exception in the c# code behind the front end completely ignores it. Here is the code i have setup, can anyone see why? I ideally want to show a js alert window on error.

protected void btnX_Click(object sender, EventArgs e)
{
throw new ApplicationException("test");
}
protected void ScriptManager_AsyncPostBackError(object sender, AsyncPostBackErrorEventArgs e)
{
ScriptManager.AsyncPostBackErrorMessage = e.Exception.Message;
}

Markup:

<script type="text/javascript" language="javascript">
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
function EndRequestHandler(sender, e)
{
window.alert(e.get_error().name);
}
</script>

<asp:ScriptManager ID="ScriptManager" runat="server" AllowCustomErrorsRedirect="true" OnAsyncPostBackError="ScriptManager_AsyncPostBackError" />