gnetworkmonitornm: Set a GError properly on an error handling path
gnetworkmonitornm: Set a GError properly on an error handling path

All the other initialisation failure paths set a GError, but this one
didn’t. Set a GError to avoid breaking the invariant that returning
FALSE should always have a GError set.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/1523

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib