Fix SQL injection and provide default responses

• the http server module now sets a default response body for 4xx and 5xx responses. It will be a text/plain description of the error like Internal Server Error
• The CRUD module was changed to validate the operator used by the client. In the past, any string would just be embedded into the SQL, allowing the client to conveniently use any advanced SQL operator, but also to embed arbitrary additional conditions or inject other SQL code. This has been fixed and the set of allowed operators limited, any operator unknown by the backend will now be rejected.