chore(api): document fine-tuning and model errors (#3964)

## Summary

Clarifies fine-tuning and model error responses while retaining existing
Python behavior.

## Changes

- Describe applicable validation, lookup, permission and rate-limit
errors.
- Preserve existing methods, return types and exception behavior.

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