We don't implement it and don't do error reporting correctly.

There doesn't seem to be a standard way to import/export PPTP connections.
Windows uses PBK files which contain multiple connections and some properties
such as remote, but not PPP parameters or cretentials. That's not very
practical. PPP settings are stored in separate CMS files:

https://bugzilla.gnome.org/show_bug.cgi?id=641254

We could also import/export PPP configurations. It wouldn't help
interoperability a lot more than if we could just export libnm keyfiles instead.