lister/tasks: Standardize return statements

The following commit adapts the return statements from both lister and their associated tasks. This standardizes on what other modules (e.g. both dvcs and package loaders) do.

It's apparently unnecessary as the listener seems to do what's necessary already [1] (but it misses tests).

One could argue that having workers (lister, loader, cooker)'s code aligned would be better in that regard (Note that we could also go the other way around)

Well, as i started this, might as well propose it...

• [1] https://forge.softwareheritage.org/source/swh-scheduler/browse/master/swh/scheduler/celery_backend/listener.py$131-139

Test Plan

tox

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Migrated from D2385 (view on Phabricator)