For queries with no results VDMS returns an error

Instead of an error VDMS should return an empty result set:
```
db.query([{'AddEntity': {'class': 'Test2', 'properties': {'x': 1}}}])
Out[6]: ([{'AddEntity': {'info': '', 'status': 0}}], [])

db.query([{"FindEntity": {'class': 'Test2', 'constraints': {'x': ['==', 2]}}}])
Out[7]: 
([{'FailedCommand': 'Transaction',
   'info': 'Failed PMGDTransaction',
   'status': -1}],
[])
```