Improve error message for mismatched copies in code blocks

[molbap]

What does this PR do?

Very tiny PR to improve error message and include the filename of the culprit in the traceback. Stumbled upon a case where attributes set before __init__ would raise this error.

cc @ydshieh @ArthurZucker

# Copied from transformers...
class MyNewModel():
    attribute = "foo"  # will break check-copies but not point to this class
    def __init__(self...

[HuggingFaceDocBuilderDev]

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[ydshieh]

Thanks for the improvement!

[amyeroberts]

Thanks for this improvement 💪

[amyeroberts]

oops -sorry - meant to hit approve