manager: handle autorestart app death before registration

Right now we rely on autorestart apps to get their death
handled when they disconnect from the bus.  This only works
if they've registered, though.

This commit makes sure the restart_app codepath gets called for
autorestart apps even if they haven't registered yet.

(This commit makes gnome-session exit if the display server couldn't
 be started)

https://bugzilla.gnome.org/show_bug.cgi?id=744752