[whisper] Clarify error message when setting max_new_tokens
Author: benniekiss
Pull request: yes

# What does this PR do?

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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.

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Addresses the discussion in #32571 


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Comments:
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 ?

the failing test is [here](https://github.com/huggingface/transformers/blob/8f8af0fb38baa851f3fd69f564fbf91b5af78332/tests/models/whisper/test_modeling_whisper.py#L1340-L1361)

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?

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

ofc! pleasure working with yall

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