Bugfixes to enable slice-to-prefab conversion to run with less warnings/errors/crashes (#768)

While trying to process all slices and levels in Automated Testing, a few bugs came up that needed to be addressed:
- [LYN-3832] TransformComponent had a field removed without updating the version number and converter, which caused a lot of excessive warnings
- SliceComponent would crash in debug builds on instantiation failures due to a null dereference that was guarded against in most but not all places
- SliceConverter now detects when nested slices exist and gracefully warns about it.
- InstanceUpdateExecutor / TemplateInstanceMapper will now immediately remove instances that are unregistered, so that any in the queue don't get processed on a subsequent tick.  This was causing crashes when the instance was destroyed before the processing occurred.  It also has a side benefit of preventing the same instance from executing multiple times.
- Minor logic bugfix to the pack close warning, the boolean check was flipped.

Also added an early-out on SetTemplateId, since this was causing some unnecessary instance queue entries.

Code/Tools/SerializeContextTools/SliceConverter.cpp

@@ -143,7 +143,7 @@ namespace AZ
                 if (packOpened)
                 {
                     [[maybe_unused]] bool closeResult = archiveInterface->ClosePack(filePath);
-                    AZ_Warning("Convert-Slice", !closeResult, "Failed to close '%s'.", filePath.c_str());
+                    AZ_Warning("Convert-Slice", closeResult, "Failed to close '%s'.", filePath.c_str());
                 }
 
                 AZ_Printf("Convert-Slice", "Finished converting '%s' to '%s'\n", filePath.c_str(), outputPath.c_str());
@@ -166,14 +166,16 @@ namespace AZ
             }
 
             // Get all of the entities from the slice.
-            SliceComponent::EntityList sliceEntities;
-            bool getEntitiesResult = sliceComponent->GetEntities(sliceEntities);
-            if ((!getEntitiesResult) || (sliceEntities.empty()))
+            SliceComponent::EntityList sliceEntities = sliceComponent->GetNewEntities();
+            if (sliceEntities.empty())
             {
                 AZ_Printf("Convert-Slice", "  File not converted: Slice entities could not be retrieved.\n");
                 return false;
             }
 
+            const SliceComponent::SliceList& sliceList = sliceComponent->GetSlices();
+            AZ_Warning("Convert-Slice", sliceList.empty(), "  Slice depends on other slices, this conversion will lose data.\n");
+
             // Create the Prefab with the entities from the slice
             AZStd::unique_ptr<AzToolsFramework::Prefab::Instance> sourceInstance(
                 prefabSystemComponent->CreatePrefab(sliceEntities, {}, outputPath));