display: Don't try to start gnome-initial setup on users check failure

Given not having users may make GDM to launch initial setup, that allows to create new users (potentially with sudo capabilities), it's better to make look_for_existing_users() to return its status and only if it didn't fail to try launch gnome-initial-setup.

We might even get rid of have_existing_user_accounts at all but I left as it might be still relevant when the async FIXME is addressed.

Fixes #642 (closed)