AddDescriptor call to non-existent set returns confusing error.

[ifadams]

Describe the bug
If a user attempts to add a descriptor to a descriptor set that does not exist/has not been created, the error returned to the client is not clear that this is the cause.

To Reproduce

Issue any AddDescriptor call targeting a non-existent descriptor set.

Expected behavior
The enduser/client should receive a clear error describing that they are attempting to add a descriptor to a non-existent set.

Additional context
Here is the error returned when a set does not exist and you attempt to add a descriptor to it, along with the JSON query that triggered it.

[{'AddDescriptor': {'set': 'persons_faces'}}]
[{'info': 'Internal Server Error: Json Exception: in Json::Value::operator[](ArrayIndex): requires arrayValue\n', 'status': -1}]