Backend work for #2524
This PR adds a second dedicated endpoint similar to `/errors`, as a
combined log endpoint would give a false impression of being the
complete crawl logs (which is far from what we're serving in Browsertrix
at this point).
Eventually when we have support for streaming live crawl logs in
`crawls/<id>/logs` I'd ideally like to deprecate these two dedicated
endpoints in favor of using that, but for now this seems like the best
solution.
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Co-authored-by: Ilya Kreymer <ikreymer@gmail.com>
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