Revert error back into warning for byte fallback conversion.
Author: Narsil
Pull request: yes

# What does this PR do?

Handles https://github.com/huggingface/transformers/pull/22264#issuecomment-1498681408

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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Comments:
_The documentation is not available anymore as the PR was closed or merged._