[bugfix] return 400 Bad Request on more cases of malformed AS data

[NyaaaWhatsUpDoc]

Description

Adds a "malformed" error value flag which we now set on parse-type failures during AS->internal conversions. These are then caught after PostInbox() and returned as 400 Bad Request errors appropriately. Also has some small tweaks here and there, and updates the go-errors package with some tweaks (which we should eventually incorporate in our own gtserror package too).

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[NyaaaWhatsUpDoc]

linter is angry, and there's an error message in a test that needs updating. a job for later kim ... :]

[NyaaaWhatsUpDoc]

closes #2145