dispatcher: don't exit with failure in case D-Bus service stops

dispatcher: don't exit with failure in case D-Bus service stops

The initsystem should stop nm-dispatcher before stopping D-Bus
service. However, on some systems that is not implemented, so
nm-dispatcher should not exit with a failure message when the
system bus disappears.
Instead just assume that D-Bus service was stopped during shutdown
and exit gracefully.

Based-on-patch-by: Jacob <jacobgodserv@gmail.com>
https://bugzilla.gnome.org/show_bug.cgi?id=751017