code: protected Mono<ServerResponse> renderErrorView(String viewName,
  			ServerResponse.BodyBuilder responseBody, Map<String, Object> error) {
  		if (isTemplateAvailable(viewName)) {
  			return responseBody.render(viewName, error);
  		}
  		Resource resource = resolveResource(viewName);
  		if (resource != null) {
  			return responseBody.body(BodyInserters.fromResource(resource));
  		}
  		return Mono.empty();
  	}
docs: Render the given error data as a view, using a template view if available or a
  static HTML file if available otherwise. This will return an empty
  {@code Publisher} if none of the above are available.
  @param viewName the view name
  @param responseBody the error response being built
  @param error the error data as a map
  @return a Publisher of the {@link ServerResponse}