Dashboard: terminate WebSocket actors on any socket-write exception.

Celluloid actors cannot self-terminate; you must do it for them.  In
this case the following could happen:

1. A socket goes into an unwritable state that isn't marked by a
   Reel::SocketError.
2. The actor receives notifications but can't process them.
3. The actor doesn't die.

The prior behavior may have contributed to the endemic memory leaks in
the dashboard.