Only invalidate session if authenticated

Fix mainmatter#721

Also update method to return `DS.InvalidError` as recommended in ember data documentation.
https://github.com/emberjs/data/blob/ffb24c5c745023fa8d5168ec6aa53051a6609751/packages/ember-data/lib/adapters/rest-adapter.js#L729-L734

addon/mixins/data-adapter-mixin.js

@@ -1,4 +1,5 @@
 import Ember from 'ember';
+import DS from 'ember-data';
 
 const { service } = Ember.inject;
 
@@ -94,8 +95,10 @@ export default Ember.Mixin.create({
   */
   handleResponse(status) {
     if (status === 401) {
-      this.get('session').invalidate();
-      return true;
+      if (this.get('session.isAuthenticated')) {
+        this.get('session').invalidate();
+      }
+      return new DS.InvalidError(['Session is not authenticated']);
     } else {
       return this._super(...arguments);
     }