protobuf-JSON rendering doesnt validate

When using JSON::Protobuf(Message&), the supplied protobuf is not checked for being properly initialized, hence e.g. required fields could be missing.

It would be desirable to have a feedback mechanism in place for this constructor - maybe this would do:

if (!message.IsInitialized()) { 
  std::cerr << "Protobuf not initialized: " << message.InitializationErrorString() << std::endl;
  abort();
}