apps: Fix exception handling in watch-appstream.py
apps: Fix exception handling in watch-appstream.py

`KeyboardInterrupt` and `SystemExit` are `BaseException`s as well, but
should not be intercepted.

Fixes CodeQL "Except block handles 'BaseException'" note.

cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit