message: Issue error if jsonschema has no root type defined
diff: @@ -198,7 +198,10 @@ class JsonSchemaGenerator : public BaseGenerator {
  bool generate() {
  code_ = "";
  - if (parser_.root_struct_def_ == nullptr) { return false; }
  + if (parser_.root_struct_def_ == nullptr) {
  + std::cerr << "Error: Binary schema not generated, no root struct found\n";
  + return false;
  + }
  code_ += "{" + NewLine();
  code_ += Indent(1) +
  "\"$schema\": \"https://json-schema.org/draft/2019-09/schema\"," +
  