remove log call
multithreading uses min_thread to determine chunking. add abort method
polling checks for duplicate exceptions (happens when multiple threads are shutdown) and does not
bother the user about them. increase exception polling frequency.
return additional exception info
better handling of user abort and status updates
add multithreading to plaintext and epub derive. fix epub text bug.
better handling of user abort. fix bug where cropper progress was not updating
improvements to process spawning, queuing, exception handling, aborting.