ember promise - handle a failed promise gracefully using model hook

I am using the loading substate of a route to show to the user that some network traffic is happening. As described here: http://ift.tt/1p28qrk

In addition to load a model in the model hook, I also saves changes during the route initialization phase. I do that in the aftermodel hook. Something like this:

afterModel () {
    var session = this.get('session');
    return session.SaveAnyUnsavedObjects();
},

//session.js
SaveAnyUnsavedObjects: function() {
    var promises = [];
    ...
    activities.forEach(function(activity) {
        if(activity.get('hasDirtyAttributes')) {
            promises.pushObject(activity.save());
        }
    }
    if(promises.get('length') > 0)
    {
        // Use RSVP.Promise.allSettled() to resolve an array of promises.
        return Ember.RSVP.allSettled(promises);
    }       
}

This works well in most cases. The problem arises when some or all my promises fail, possibly due to offline. So I want to add a "then"-handler to my promises, so can execute some code when the promises are done. How can I add a then-handler and at the same time keep the loading substate?

In other words, how can I return a promise to the model hook or aftermodel hook as soon as the ajax call starts and at the same time have code that handles the result of the promise?