[whisper] Clarify error message when setting max_new_tokens

[benniekiss]

What does this PR do?

This PR clarifies the error message returned when the length of the prompt tokens + max_new_tokens exceeds the max_target_positions of the whisper model.

Addresses the discussion in #32571

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@ylacombe

[ylacombe]

Thanks for opening this PR @benniekiss !
There's weirdly a Whisper failing test. Would you like to first rebase the PR branch, and then we'll see what we can do if it's still there ?

[benniekiss]

the failing test is here

It seems that the old error message is hardcoded in the test. Should I go ahead and update it to the new message so they are in sync?

[ylacombe]

Oh right, let's do this! It's a bit weird indeed, but let's just correct the test

[ylacombe]

Will merge once it's all green ! Thanks for iterating !

[benniekiss]

ofc! pleasure working with yall

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