We currently just call sassc, but don't check its return value. That
means as long as sassc is available, the script (and therefore the
newly added dist CI job) will succeed.
Make sure we fail on failure.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1974>
Since commit 9ef9a5ff8a, man pages are generated at dist time to reduce
build dependencies when building from a released tarball.
Do the same for the stylesheets to also remove the sassc dependency.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1555>