When using the client syntax (so using a query like db.something.find(...)) with a ill formed JSON query, we have an obscure error about using db instead of a valid JSON, whereas the issue is not about the db part.

We should try to improve the parser so that it reports an understandable error in the following cases:

• the command used is not supported (command as in find)
• the JSON query is not valid (but it does not make the db part invalid)