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/ComposerBody.js

@@ -102,4 +102,12 @@ export default class ComposerBody extends Component {
    */
   onsubmit() {
   }
+
+  /**
+   * Stop loading.
+   */
+  loaded() {
+    this.loading = false;
+    m.redraw();
+  }
 }