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