code: public Javalin error(int statusCode, @NotNull ErrorHandler errorHandler) {
          servlet.getErrorMapper().getErrorHandlerMap().put(statusCode, errorHandler);
          return this;
      }
docs: Adds an error mapper to the instance.
  Useful for turning error-codes (404, 500) into standardized messages/pages
  
  @see <a href="https://javalin.io/documentation#error-mapping">Error mapping in docs</a>