Re-implement rclone presence check

[williamjohnstone]

AutoRclone currently does not check for the presence of rclone, there is a function check_rclone_exists() inside helpers.py but currently it just returns true, iirc I did this because I had issues with the distutils package and just hardcoded so I could continue testing something else. This check is pretty critical so this needs to be re-implemented properly, if anyone wants to pick this up you can, otherwise I'll get around to fixing this at some point.