luxi: do not handle KeyboardInterrupt

With the current code, it's possible to mistake a ^C for a protocol
error:

node1# gnt-job info 221691
[press ^C]
Unhandled protocol error while talking to the master daemon:
Error while deserializing response:

(and note empty error message).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>