Improve LLaMA tokenizer error when vocab is missing: suggest installi…

[predMoT]

…ng mistral-common\n\nAdd unit test to assert helpful message.

What does this PR do?

Fixes # (issue)

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[github-actions]

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

run-slow: llama

[Rocketknight1]

cc @ArthurZucker @itazap we're getting a lot of PRs/issues about loading tokenizers from Mistral checkpoints!