Improve client XHR error handling

The default XHR error handler produce an alert which is appropriate to the response status code. It can be overridden per-request (by specifying the `errorHandler` option) so that the alert can be suppressed or displayed in a different position (e.g. inside a modal).

ref #118

js/lib/Store.js

@@ -79,10 +79,11 @@ export default class Store {
    *     Alternatively, if an object is passed, it will be handled as the
    *     `query` parameter.
    * @param {Object} [query]
+   * @param {Object} [options]
    * @return {Promise}
    * @public
    */
-  find(type, id, query = {}) {
+  find(type, id, query = {}, options = {}) {
     let data = query;
     let url = app.forum.attribute('apiUrl') + '/' + type;
 
@@ -94,11 +95,11 @@ export default class Store {
       url += '/' + id;
     }
 
-    return app.request({
+    return app.request(Object.assign({
       method: 'GET',
       url,
       data
-    }).then(this.pushPayload.bind(this));
+    }, options)).then(this.pushPayload.bind(this));
   }
 
   /**