Ignore POLL events for routers in ERROR status.

Doing this check here requires the worker to keep up with router state, which it doesn't do right now. I would rather put this down inside Automaton.send_message(), which will have access to the VmManager. That will also let us look at a local state variable instead of making a neutron API request for every event. 

If you add an ERROR status to vm_manager.py (we have DOWN, BOOTING, etc. at the top of the file), I can update VmManager to use that status when appropriate in the changes I'm working on separately.