ASP.NET ToolkitScriptManager AsyncPostBackError - throw exception from Content Page

Question

Wonder if anyone can help. I'm trying to implement the ToolScriptManager OnAsyncPostBackError.

The ToolkitScriptManager is on the Masterpage and I've set the OnAsyncPostBackError property:

<ajax:ToolkitScriptManager ID="ToolkitScriptManager" 
    runat="server" EnablePartialRendering="true" 
    OnAsyncPostBackError="ScriptManager_AsyncPostBackError">
</ajax:ToolkitScriptManager>

On my content page I'm catching an exception and, logging it and then throwing it

    private void LogError(Exception ex, bool full)
    {
        ...
        _presenter.LogError(this, error, ex);

        throw new ApplicationException(
            "Error Occured",
            ex
            );

    }

where it then gets picked up by the handler on the Master Page

    protected void ScriptManager_AsyncPostBackError(object sender, AsyncPostBackErrorEventArgs e)
    {
        if (e.Exception.Data["GUID"] != null)
        {
            string _message =
           (
                e.Exception.InnerException != null
            ) ? e.Exception.InnerException.Message : e.Exception.Message;

            ToolkitScriptManager.AsyncPostBackErrorMessage = _message +
                " When reporting this error use the following ID: " +
                e.Exception.Data["GUID"].ToString();
        }
        else
        {
            ToolkitScriptManager.AsyncPostBackErrorMessage =
                "The server could not process the request.";
        }
    }

This all works as expected, I'm picking up the error via the PageRequestManager EndRequest and displaying on the page

The problem is that I'm getting an Application_Error in my Global.asax stating that an Exception of type 'System.Web.HttpUnhandledException' was thrown.

I kinda get it I think, cause I'm throwing the exception which is not caught by a catch I'm assuming?!

Any advice on what I'm missing, or where in fact I'm going wrong?

Thanks in advance!

Ian