Handle 500+ status codes when requesting a SoUD sync

[rtibbles]

Summary

• Fixed some assertions in tests
• Added test for handling ConnectionError in request
• Fixed issue where using requests.exceptions.ConnectionError made Python report that it was not derived from BaseException
• Handle 500+ status codes from the queue endpoint by initiating ttl back off

Reviewer guidance

Tests cover the changes?

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[jamalex]

I don't know the scheduling code well enough to say anything definitive around how the tests work, but the code change itself seems wise (though I added a comment about raise_for_status as a possible alternative pattern, noting that it raises requests.exceptions.HTTPError instead, so we'd then need to catch that too).

[jredrejo]

LGTM, thank you!