Consider only marking shards as unavailable for UnavailableInvalidChannel() exception instead of most 4xx errors.

[dholth]

Checklist

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What is the idea?

conda's http error handling should be refactored and simplified, but #844 copies the repodata gateway's advertised behavior of not checking for repodata_shards for 7 days on most 400 errors.
It might make sense to trap only UnavailableInvalidChannel() which is a narrower exception.