Handle AJAX errors when updating content

With the change in implementation in the previous commit, any error
(eg server responds with `500`) would cause the page to not be updated
again.

This is better than the previous implementation, whereby the browser
would re-request as fast as it could until it got a successful response.

This commit handles errors by clearing the render queue if the server
returns an error. So:

- Any updates that would have been performed based on this request are
  dropped
- Subsequent updates will be attempted as if it was the first load of
  the page, ie after a delay of `x` seconds

app/assets/javascripts/updateContent.js

@@ -17,11 +17,16 @@
     while(queue.length) queue.shift()(response);
   };
 
+  var clearQueue = queue => (queue.length = 0);
+
   var poll = function(renderer, resource, queue, interval) {
 
-    if (queue.push(renderer) === 1) $.get(
-      resource,
+    if (queue.push(renderer) === 1) $.ajax(
+      resource
+    ).done(
       response => flushQueue(queue, response)
+    ).fail(
+      () => clearQueue(queue)
     );
 
     setTimeout(