Image Extraction completely aborts the conversion if an unexpected file extension is encountered

[Lunalicious]

Hey hey,
first of all, very nice work! This project really helps me with my current work.
However, I have encountered an error when extracting images.

If you want to parse a pptx file that has MPO images (or any other unsupported file format), python-pptx will throw a ValueError which does not get caught.

I'll put up a PR that fixes that if you're fine with that :)

[Lunalicious]

Accidentally closed the wrong PR, so I put up a new one.