When 'nm-dispatcher' is not running because its systemd service
'NetworkManager-dispatcher.service' is not enabled, any calls to the dispatcher
will fail with an error of typ DBUS_ERROR:DBUS_GERROR_REMOTE_EXCEPTION (32):

  "Unit dbus-org.freedesktop.nm-dispatcher.service failed to load: No such file or directory."

This clutters the logfile with warnings, although the user probably
disabled the service on purpose.

Special case this particular (recurring) failure and downgrade the warning
to debug level.

Signed-off-by: Thomas Haller <thaller@redhat.com>