Telerik did not implement custom error handling for RadAjax , because the Microsoft 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.
Set the AsyncPostBackErrorMessage property, which is the error message that is sent to the browser.
Take advantage of the PageRequestManager class which enables you to handle events in the client page life cycle and to provide custom event handlers that are specific to partial-page updates.