Insert of duplicate name of descriptor set does not return meaningful error with Neo4J

**Describe the bug**
If two descriptor sets of the same name are inserted into VDMS while using the Neo4J handler, the second insert fails (which is correct behavior), but no error is returned to the client.

**To Reproduce**

Insert 2 descriptor sets into VDMS using the Neo4J handler.

**Expected behavior**

The second insert should fail, but also return an error to the client. Currently it returns a null (none) value.

**Screenshots**

```
ifadams@anchor:~/gitrepos/vdms_bench$ python3 neo_knn_find.py
([{'NeoAddDescriptorSet': {'status': 0}}], [])
ifadams@anchor:~/gitrepos/vdms_bench$ python3 neo_knn_find.py
(None, [])
```
Note second return is what occurs when a duplicate descriptor set is inserted.