dwarfdump: handle empty unit in dump_line

[philipc]

If an error occurs when dumping line info for a unit, then continue to the next unit instead of exiting.

Fixes #329

[philipc]

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[philipc]

We probably should do something like this for every while loop in dwarfdump, and the io::Error handling is ugly...

[fitzgen]

yeah, it's kind of annoying, but it does precisely describe what we want, and I'm having a harder time imagining how it would be particularly better :-/

[philipc]

Maybe we should just be exiting immediately on IO errors. I guess we could hack that in the impl From<io::Error> for Error.