Trying to cancel a save event on the client, based on a property failing a particular validation, but am getting a 500 error dialog in the backoffice.

Any ideas on how to handle this? Preference is to keep it client side as it's part of a suite of httpinterceptors managing other simple business logic.

function contentSave($q) { 
    var canceller = $q.defer();

    return {
        request: function (request) {
             // check property, set cancel = true if invalide

             if  (cancel) {
                 request.timeout = canceller.promise;
                 canceller.resolve();

                 // save is cancelled, but ysod dialog is triggered, with no error message as the request was never sent.
             }
             return request || $q.when(request);
        }
    }
}