Prevent acd from trying to use non-existing pyacd methods when trying to authenticate when a client is still running.

[cambrant]

pyacd.Session.is_logined is renamed to is_logged_in(), and pyacd.Session.is_valid() is removed, thus unnecessary. The try block will raise an exception when trying to assign values to ACDFS.session is None. If that happens, we don't have to check if the session exists at all.

[handyman5]

Thanks so much!