modified process_file() for a better "--no-overwrite" experience

[kai-modrzok]

When I initially create contact-sheets in a video-folder the recursive functionality from vcsi works fine. But when I only want to create the contact-sheets for NEW videos - vcsi works not at it's best.

I can use the "--no-overwrite" mode, but it still parses the video. Also it find's the contact-sheets itself and tries to parse them as well (which results in errors). If I then use --ignore-errors it works better. But then I don't see errors anymore ;-)

So I slightly adjusted the process_file() method and moved the file check to the front of the method. Now i can just say "vcsi --no-overwrite ." in a folder over and over again and still get potential errors during contact-sheet generation.

[coveralls]

Coverage increased (+0.2%) to 32.109% when pulling 73e6b28 on kai-modrzok:feature/recursive-enhancements into eb436cc on amietn:master.

[coveralls]

Coverage increased (+0.2%) to 32.109% when pulling 73e6b28 on kai-modrzok:feature/recursive-enhancements into eb436cc on amietn:master.

[amietn]

Thank you for contributing, much appreciated.