launcher: Print a more descriptive error message when we don't get a VT
launcher: Print a more descriptive error message when we don't get a VT

weston-launch has two supported use cases now: either launch from
and VT login as a regular user (running within that session) or
from an init script (systemd or such) with the -u option to create
a session for the specified user.  Running from within X or ssh is
not possible.  It's still possible to run weston as root from X or ssh
but that's strictly a development/debug/test feature.

https://bugs.freedesktop.org/show_bug.cgi?id=69727

Gnurou/weston,Fantu/compositor-spice,mchalupa/weston,xorgy/weston,giucam/weston,jonnylamb/weston,jonnylamb/weston,krezovic/weston,udoprog/weston,eyolfson/weston,sir-murray/weston,kwm81/weston,eyolfson/weston,Gnurou/weston,udoprog/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,giucam/weston,sir-murray/weston,kwm81/weston,xorgy/weston,mchalupa/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,krezovic/weston