Instead of sub-process, you may want to consider python-gphoto2.
See this discussion:
More examples.
I do not know what the (-32763) refers to.
BTW, I saw your issue on GitHub. What is the answer to getting the error log?
Coincidentally, I was just talking to @jcasman about spending more time with the Python and USB API tests. We normally focus more time on the WebAPI, but there are many people using the USB API now.