Fixed bug: repeated meeting ID crashed script

Fixed bug: repeated meeting ID crashed script

The script was not able to handle the case where the same meeting ID was
encountered more than once.  This is because each meeting ID is a
directory in the downloads directory, and creating a directory that
already exists is an error in Python.

While I was at it, I also put in a check for existing files so that
restarting the downloader will not start over from the beginning again.
This makes it easier to restart, but could lead to a file being
partially downloaded and then skipped over subsequently, but it is a
better place to be than restarting a download of hundreds of videos from
scratch.