Currently the generic handling of ganeti errors in cli.py (GenericMain
and FormatError) only handles the core ganeti errors, and not the client
protocol errors (which live in a separate hierarchy).

This patch adds handling of luxi errors too, and also adds another luxi
error for the case when the master is not running. This gives us a nice:

  gnta1:~# gnt-node list
  Cannot communicate with the master daemon.
  Is it running and listening on '/var/run/ganeti-master.sock'?

error message instead of a traceback.

Reviewed-by: amishchenko