fix: improve AutoTokenizer error message for missing vocab files (#41…
Author: cwarre33
Pull request: yes

…553)

Add clear error message when LlamaTokenizer fails to load because vocab_file is None, which typically happens when optional dependencies like mistral-common are not installed.

This provides users with:
- Clear explanation of why the error occurred
- Specific guidance for installing mistral-common for Voxtral/Mistral models
- General troubleshooting advice for other models

Before: Users would see a cryptic 'TypeError: not a string' from sentencepiece
After: Clear ValueError with actionable guidance

Fixes #41553

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Fixes # (issue)


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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: llama

I'm not sure you committed the files you intended to here!