Handle Python parsing errors better

[simonw]

Right now if any of the inputs to symbex include Python that cannot be parsed by the ast module the whole process crashes with a noisy error.

[simonw]

I think files with parse errors in are logged to stderr, maybe with a -s/--silent option to suppress them.

[simonw]

I'm going to drop the -s shortcut since I want to use that for something else:

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