Reports sync failed notification.

Displays bar with error message and a retry button when sync fails. Closes #376 
Couldn't this method body be like:

```
if (isLoading || !UserVisibleHint || controller == null) {
    return;
}
controller.Data = null;
EnsureLoaded();
```

Better call the `EnsureLoaded` function than copy-paste the content of it.