[summarization example] better error message
https://github.com/huggingface/transformers/pull/15125 introduced a new required arg. 

This PR improves on the error message that is easier to act on as the original is not actionable unless one intimately knows the script.

@sgugger 
Wait, this is an optional argument that only needs to be set if MBart is used as a model.

There is an assert later on in the script, so this test should just be removed from the postinit.<|||||>you're saying it was invalid logic in the first place in the original PR, correct?

Need more tests then since the issue wasn't caught by normal live tests?<|||||>There is a [test](https://github.com/huggingface/transformers/blob/1eb40338ac41a2ffbb2137216de3bb63bb739aad/examples/pytorch/test_examples.py#L350) that catches the problem, it's just marked as slow.<|||||>ah, right, I forgot about the slow ones! thank you, Sylvain.

So closing this as it is no longer relevant due to revert in https://github.com/huggingface/transformers/commit/96881729ce83cfc8e5fa04c903ee4296ad17cfbb