[bugfix] fix media limit reader check

[NyaaaWhatsUpDoc]

Description

So it turns out that doing an empty read to check for io.EOF only works with some readers. This fixes that! And improves some of the errors we return to API callers when local media limits have been reached.

This is a partial fix for #3362 which encompasses 2 issues.

Checklist

• [x] I/we have read the GoToSocial contribution guidelines.
• [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• [x] I/we have not leveraged AI to create the proposed changes.
• [x] I/we have performed a self-review of added code.
• [x] I/we have written code that is legible and maintainable by others.
• [x] 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.
• [x] I/we have run tests and they pass locally with the changes.
• [x] I/we have run go fmt ./... and golangci-lint run.