Errors in parsing should throw an exception

[mfherbst]

I find it a bit counterintuitive that erroneous XYZ files only print something (to stderr I believe) but otherwise simply return an empty list. Instead I would have expected an exception with the error to be raised.

[mfherbst]

Just to show a case in point: #35 was initially obfuscated by this behaviour and took me a bit to figure out what's the problem.

[jameskermode]

As you may have seen I'm very belatedly catching up on these issues. I agree with this one as well, but it will require changes in the underlying C library. I've opened an issue at libAtoms/extxyz#14