fix: add clear error message when mistral-common is missing for AutoTokenizer loading Voxtral

[junjunjd]

• Add clear error message when mistral-common is missing for AutoTokenizer loading Voxtral
• Add unit test

What does this PR do?

Fixes # (issue)

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[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: auto, voxtral

[Rocketknight1]

cc @vasqu we have quite a few PRs for this now and should probably merge one! This one seems clean, but #41592 has been open for longer

[vasqu]

Sorry about that one, imo we should prioritize #41592, left a comment there

This PR will only work for voxtral, we should make this as general as possible as in the other PR

[Khansa435]

Hi @Rocketknight1 @vasqu . I've worked on @vasqu suggestion and did changes .My PR #41592 passes all tests. One test (build_pr_documentation) is pending and rest are all clear.