The ConnectInteractive() -> Connect() fallback code doesn't work, because
_connect_internal() changes the state to NM_VPN_SERVICE_STATE_STARTING before
checking if it can implement ConnectInteractive(), and then when the Connect()
call comes in, the VPN is not in STOPPED or INIT, so it returns an error.

The commit moves setting state to STARTING after the ConnectInteractive() check
availability, in the plugin. We introduce new plugin error and set it when the
the plugin does not implement ConnectInteractive(). NetworkManager uses this
error for ConnectInteractive() -> Connect() fallback.

https://bugzilla.gnome.org/show_bug.cgi?id=708255