give some user feedback on session startup failures

In /usr/share/lightdm-remote-session-x2go/x2go-session, there could be more verbose interaction with the user in case an X2Go session does not come up.

E.g., we could have a timer wrapped around the pyhoca-cli call and see how long the session lasts. If it dies to early (e.g within 2 secs), something probably went wrong with pyhoca-cli.
