Don't exit when local display fails to start
    
    It doesn't really make a lot of sense to exit when the
    local display can't be started.  Exiting won't leave the system
    in any better shape that it would be with gdm running in the
    background.  On the other hand it makes it so that XDMCP and other
    services can't run headless.  Ideally we'd have some kind of
    smart detection of when local hardware is available and when it
    changes.  And smarter fallback code for when the hardware is
    available but the display can't be started due to some error.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=342397