Improve error message for missing VPN plugins

There are two cases where this error will be shown:

1) plasma-nm is built without support for that VPN because of missing optional dependencies.

2) NetworkManager is missing a runtime package for that VPN type, e.g. for openconnect one needs network-manager-openconnect on Ubuntu

The current error message is quite unhelpful since it doesn't tell what kind of thing is missing. Try to improve that a bit.

Not that this is the pinnacle of UX. I don't think there's a nice and easy way to guide the user to installing the right package.