No QJsonDbWriteRequest::ErrorCode value for UUID/version conflict

In the use case "use reproducible hashes for UUIDs as poor man's substitute for primary keys", we lack an error code to tell that object creation has failed due to the UUID conflict (which means that another client has created the object, so we need to reuse it). This makes reliable concurrent creation of objects that need to be unique with regard to some key even more tedious and error-prone.

In general, everyone who has to deal with concurrent modification of objects in the RejectStale resolution mode probably needs this error code to implement graceful conflict resolution.