chore(api): document batch error responses (#3961)

## Summary

Documents applicable Batch API errors while preserving Python exception
behavior.

## Changes

- Describe invalid requests, missing batches, and cancellation
conflicts.
- Clarify applicable rate-limit and service-error responses.

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