Add failure tests in swh-storage

Not all writes are transactional:

• we publish to Kafka before committing in postgresql
• writes in cassandra are not transactional

we should write tests that cover at least non-crash failure scenarios, eg by monky-patching psycopg2 or cassandra-driver (or CqlRunner) to raise an unexpected exception.

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Migrated from T2434 (view on Phabricator)