RadControls for ASP.NET AJAX
We did not implement our own error handling for RadAjax, as the MS AJAX framework already takes care of that. During partial-page rendering, you can handle errors by doing the following:
Set the AllowCustomErrorsRedirect property, which determines how the custom error section of the Web.config file is used when an error occurs during an asynchronous postback.
Handle the ScriptManager control's AsyncPostBackError event, which is raised when there is a page error during an asynchronous postback.
Set the AsyncPostBackErrorMessage property, which is the error message that is sent to the browser.