swh-graph CI hangs badly when py4j doesn't find needed files

The CI runs for swh-graph might hang badly on test_api_client.py, here's the most recent exampe.

This specific issues is solved now, but it really shouldn't happen that test running hangs, see below for an analysis by @olasd on how to avoid that exception aren't forwarded, blocking test abortion.

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Migrated from T2055 (view on Phabricator)