Report error for unknown options.

Why didn't I add this initially?

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Write error message to SRT in case of OCR failure.

This will make it easier for the user to deal with them and won't change
the subtitle numbering.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>