x11/sddm: make "user session" more resilient

It's possible to install SDDM with very little other X11 baggage,
like xterm. In such a minimal install situation, SDDM will still
come up and offer "User Session" -- and that session will fail,
because there's nothing to run.

Depend on xmessage, and then fall back to xinit's default script
and if that isn't there, show the user a message instead of
just sitting there with a black screen and an X cursor.

PR: 256648
Reported by: Graham Perrin