Improve error handling somewhat

- Fix composer crashing/not showing alert on error
- Make a general ValidationException which takes an array of field ⇒
messages to be outputted nicely by the API

js/forum/src/components/DiscussionComposer.js

@@ -108,7 +108,11 @@ export default class DiscussionComposer extends ComposerBody {
         app.cache.discussionList.addDiscussion(discussion);
         m.route(app.route.discussion(discussion));
       },
-      () => this.loading = false
+      response => {
+        this.loading = false;
+        m.redraw();
+        app.alertErrors(response.errors);
+      }
     );
   }
 }