Queueitem should be moved to error queue if the distribution package does not exist anymore

There can be a case when the distribution package has been processed but the queueitem has not been removed properly and is being re-processed. In such a case (when the associated distribution package is null), the queue item is simply removed. Thus, the user does not have the ability to check what happened. If a queueitem was sent to processing without the corresponding distribution package, then something must have errored, either the package was distributed successfully but the queueitem was not removed successfully by the QueueProcessor or some other code bug. In such a case we can consider moving the queueitem to error queue instead of just removing it with an error log.

cc - ashishc