code: protected RefProperty registerErrorModel(Swagger swagger) {
          String ref = Error.class.getSimpleName();
          if (swagger.getDefinitions() != null && swagger.getDefinitions().containsKey(ref)) {
              // model already registered
              return new RefProperty(ref);
          }
  
          ModelImpl model = new ModelImpl();
          swagger.addDefinition(ref, model);
  
          model.setDescription("an error message");
  
          model.addProperty("statusCode", new IntegerProperty().readOnly().description("http status code"));
          model.addProperty("statusMessage", new StringProperty().readOnly().description("description of the http status code"));
          model.addProperty("requestMethod", new StringProperty().readOnly().description("http request method"));
          model.addProperty("requestUri", new StringProperty().readOnly().description("http request path"));
          model.addProperty("message", new StringProperty().readOnly().description("application message"));
  
          if (settings.isDev()) {
              // in DEV mode the stacktrace is returned in the error message
              model.addProperty("stacktrace", new StringProperty().readOnly().description("application stacktrace"));
          }
  
          return new RefProperty(ref);
      }
docs: Manually register the Pippo Error class as  Swagger model.
  
  @param swagger
  @return a ref for the Error model