Oban: more improvements

Analyzing errors, improving cases where we can avoid needlessly retrying, etc. Making Pleroma more polite.

Publisher jobs will not retry if the error received is a 400

I have been observing that Mostr.pub keeps throwing 400s and the AP specs have 400 reserved as a valid placeholder response, but hasn't (yet) decided if it's meant to be a recoverable error.

https://www.w3.org/wiki/ActivityPub/Primer/HTTP_status_codes_for_delivery