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/forum/src/components/ChangePasswordModal.js

@@ -42,8 +42,8 @@ export default class ChangePasswordModal extends Modal {
       url: app.forum.attribute('apiUrl') + '/forgot',
       data: {email: app.session.user.email()}
     }).then(
-      () => this.hide(),
-      () => this.loading = false
+      this.hide.bind(this),
+      this.loaded.bind(this)
     );
   }
 }