Fix invalid SUCCESS in [Ts]DataSvc::retrieveEntry

When an entry in the middle of a transient store path is not valid and there is no "data fault handler enabled" we obtain a StatusCode::SUCCESS out of [Ts]DataSvc::retrieveEntry but the object pointer is nullptr.

This happens because when we try to get the missing level and we fail we return the current value of status which at that point cannot be anything else than SUCCESS.

This MR forces the return value to be a failure code.