This fixes a problem with deleting an unmounted package instance.
If you added and mounted, say, a subsite instance, unmounted and then
attempted to delete it through sitemap->Manage unmounted applications, you
would get an error. It seems site_node::get_node_id_from_object_id doesn't
even fail for an unmounted package, returning the empty string in this case,
and the catch test doesn't trip. Now we explicitly test for the node_id
being set to a sensible value.
