Display visual cue on icon when SABnzbd can't be reached

[starkcolin]

It would be useful to have a visual indicator on the icon when SABConnect++ can't communicate with SABnzbd. Display a red X in the same location as the number of active downloads would work well. I have been having issues with SABnzbd locking up and it would be useful to be able to see at a glance when it has become inaccessible.