FindDescriptors will fail when not never specified in any AddDescriptor call.

query: [{'FindDescriptor': {'_ref': 1,
'k_neighbors': 3,
'results': {'blob': True},
'set': 'test_emb_50'}}]
response: [{'FailedCommand': 'Transaction',
'info': 'Failed PMGDTransaction',
'status': -1}]