chore(api): document files and uploads error responses (#3960)

## Summary

Documents applicable request, missing-file, upload-state and storage
errors for Files and Uploads. Python exceptions are unchanged.

## Changes

- Describe Files lookup and validation failures.
- Clarify expired-upload recovery and applicable request-rate limits.

Co-authored-by: markstuart-oai <323302876+markstuart-oai@users.noreply.github.com>