logind: remove an obscure dbus error from GetSessionByPID(0) and friends

logind: remove an obscure dbus error from GetSessionByPID(0) and friends

GetSessionByPID(0) can fail with NO_SESSION_FOR_PID.  More obscurely, if
the session is abandoned, it can return NO_SUCH_SESSION.  It is not clear
that the latter was intended.  The message associated with the former,
hints that this was overlooked.

We don't have a document enumerating the errors.  Any specific
error-handling in client code, e.g. translated messages, would also be
liable to overlook the more obscure error code.

I can't see any equivalent condition for GetUserByPID(0).  On the other
hand, the code did not return NO_USER_FOR_PID where it probably should.
The relevant code is right next to that for GetSessionByPID(0), so it will
be simpler to understand if both follow the same pattern.