Improve error handling and messages
Improve error handling and messages

dlvideos.py:
Keyboard interrupts (^C) are now more cleanly handled and spew fewer errors.
If writing done.json fails, a warning is now printed.
Error messages are terser and (hopefully) more direct.

general changes:
Some strange variable names are fixed, at least until I freak out and change them again.
(videosJson_file? What was I thinking?)
Added a few comments here and there.

drkitty/edopaffy