If you want to show an error when an AJAX request fails, that failure constitutes a change in application state, which should be reflected in a server action. This action would end up in an ErrorStore, which a DisplayErrors component would be listening to somewhere. If you show an alert dialog (for example), the user closing that dialog would also constitute a change in application state, as the error has been dismissed, which would be reflected in a view action, which would eventually get to the ErrorStore and remove that error.