At the moment if logind can fail in two ways when
asking the session associated with the current pid:

1) ENOENT, the process isn't part of a registered session
2) ENODATA, the mechanism for checking which session a
process is registered with isn't working.

If we hit the second case then wefail loading the user manager
entirely.  This leads to the dbus proxy associated with a user
from loading and the user getting stuck with defaults like a
NULL xsession and systemaccount=TRUE

This commit changes the behavior for the second case to be
like the first. Namely, to accept there's no associated
session and carry on as best we can.