For an issue observed in this thread: https://discuss.huggingface.co/t/error-generating-deep-rl-course-certificate/177241
---
## Summary
This PR makes the Deep RL Course Certification Space easier to rebuild and improves the error message around the certificate writeback step.
Changes:
- Pin Python to 3.10 because a fresh build currently uses Python 3.13 and `Pillow == 9.4.0` fails to build there.
- Pin `huggingface_hub<1` as a temporary compatibility fix because the Space imports the legacy `Repository` class.
- Add input validation for empty username / name fields to avoid accidentally scanning all matching public Hub models.
- Replace the generic Gradio failure around the certified-users writeback step with a clearer message and a logged exception.
This PR intentionally does **not** return a certificate when the writeback step fails, because `certified_users.csv` may be intended as the official issuance record. It only makes the failure explicit instead of surfacing a generic `…