[bugfix] improved authenticate post inbox error handling

[NyaaaWhatsUpDoc]

Description

updates the failed auth error handling on the post inbox route to better handle returned errors, previously we were masking a lot with just "not authenticated" which wasn't the most useful in the logs :D

Checklist

• 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 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.
• 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]

@daenney have fixed the test now and added some extra code comments explaining the odd ish logic going on

[daenney]

Awesome. Thanks for adding the comments, that's great!