When we are shutting down the lxc network, we should not fail when
things go wrong, as that only makes it harder to clean up later.

See https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1429140 in particular
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>