Installation should refuse to continue if /etc/wireguard exists

I just had to work around an issue where uninstalling and reinstalling this script left the keys generated from a previous installation, in a new installation. As a result, none of the keys generated, worked with the new install.

The script installation should complain, and refuse to continue, if previous configuration files exist. Alternatively, it should warn the user loudly about problems with the previous install, including saying loudly that newly generated keys won't work.