Check if we have write access to the target folder of wifi-completion.ba...

...sh before installing it. This comes in handy in case we are installing this wifi module into a virtualenv, in which in most cases we don't want to use superuser permissions.

I've to be honest: I don't know if this is the proper way of achieving my goal... so I've no problem if you reject this PR ;-), I just needed a clean installation via pip into a virtualenv for me.
Np - this happens! - I'm using the network-manager directly by alter&creating files in /etc/NetworkManager/system-connections. To scan for available networks I'm parsing the nmcli command.