weston-launch: quit if -t without -u

weston-launch: quit if -t without -u

setup_tty() function uses the tty argument for choosing the tty/VT only
if wl->new_user (the -u option) is given. If the tty option is given
without -u, it will only be used for misleading error messages.

To make it clear to the user that -t without -u does not work the way
one might think, let weston-launch exit with an error in that case.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>