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

gio/gnetworkmonitornm.c

@@ -309,6 +309,8 @@ g_network_monitor_nm_initable_init (GInitable     *initable,
 
   if (!name_owner)
     {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                   _("NetworkManager not running"));
       g_object_unref (proxy);
       return FALSE;
     }