Implement handling of luxi errors in cli.py

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