You need to have the ppa enabled in your sources before running ppa-purge or if quiting a ppa-purge attempt. (1st thing ppa-purge does is disable the ppa to be removed
So open your software sources (software-properties-gtk), re-enable the ppa & then update your sources -
sudo apt-get update
There is likely no way ppa-purge is going to do a clean, complete job so before using i'd strongly advise to install synaptic.
Then run your ppa-purge command, your best bet may be to accept the 1st solution, Y, & then answer Y to any other presented until it does it's thing.
Once finished Do Not log out or restart
Open synaptic, click 'Reload', then search gnome-session & mark for install if it's not installed. Then search unity & mark unity for install or re-install.
Additionally search nautilus & mark libnautilus-extension1 for reinstall, check & make sure that nautilus & nautilus-data are installed, if not mark also & apply.
If desired you could search ubuntu-desktop and mark for reinstall in lieu of above, just make sure the packages I've mentioned are installed before you log out or restart
Also note that unity-2d may need to be reinstalled