If a PlanStage encounters a runtime failure, it currently is required to allocate a special WorkingSetMember representing a Status object and to return StageState::FAILURE. This forces all PlanStage implementations to be prepared to explicitly handle a FAILURE status from their children, which results in lots of boilerplate. It would be simpler to fail by throwing an exception, like we do if a query fails in DocumentSource execution code. This will also allow us to delete the undesirable code that packs a Status object into a Document.