[chore] Refactor federatingDB.Undo, avoid 500 errors on Undo Like

[tsmethurst]

Description

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This pull request does a bit of refactoring of undo.go in the federatingdb package, specifically to bring it in line with other more modern files in that package like reject.go.

Thing 1: Don't return 500 if we just can't get something because we never had it stored. This was happening frequently when Misskey and Friendica instances were sending us Undo's of Likes that we'd never processed, leading to annoying log messages like this:

timestamp="16/09/2024 07:58:45.971" func=server.init.func1.Logger.13.1 level=ERROR latency="39.358876ms" userAgent="Friendica 'Yellow Archangel' 2023.12-1542; https://social.gl-como.it" method=POST statusCode=500 path=/users/tobi/inbox clientIP=xxx.xxx.xxx.xxx pubKeyID=https://social.gl-como.it/profile/valhalla#main-key errors="Error #01: PostInboxScheme: error calling sideEffectActor.PostInbox: Undo: error undoing like: undoLike: error converting ActivityStreams Like to fave: getASObjectStatus: error getting object account from database: sql: no rows in result set\n" requestID=nhypkych040013sdj0rg msg="Internal Server Error: wrote 33B"

Now we just ignore such messages + return 202.

Thing 2: Have the federatingactor actually check for possible gtserror.WithCode instances, and use those instead of falling back to 500. I think we just forgot to hook this up, so we were returning lots of fancy different types of WithCode errors, but just squashing them all into 500 anyway :') Oops.

No real logic changes aside from that!

Checklist

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• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
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• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
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