build: Check sassc errors when generating stylesheets

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>

meson/generate-stylesheets.py

@@ -16,4 +16,4 @@ for stylesheet in stylesheets:
     stylesheet_path = PurePath(stylesheet)
     src = PurePath(sourceroot, stylesheet_path.with_suffix('.scss'))
     dst = PurePath(distroot, stylesheet_path)
-    subprocess.call(['sassc', '-a', src, dst])
+    subprocess.run(['sassc', '-a', src, dst], check=True)