code: protected Mono<ServerResponse> renderDefaultErrorView(
  			ServerResponse.BodyBuilder responseBody, Map<String, Object> error) {
  		StringBuilder builder = new StringBuilder();
  		Date timestamp = (Date) error.get("timestamp");
  		Object message = error.get("message");
  		Object trace = error.get("trace");
  		Object requestId = error.get("requestId");
  		builder.append("<html><body><h1>Whitelabel Error Page</h1>").append(
  				"<p>This application has no configured error view, so you are seeing this as a fallback.</p>")
  				.append("<div id='created'>").append(timestamp).append("</div>")
  				.append("<div>[").append(requestId)
  				.append("] There was an unexpected error (type=")
  				.append(htmlEscape(error.get("error"))).append(", status=")
  				.append(htmlEscape(error.get("status"))).append(").</div>");
  		if (message != null) {
  			builder.append("<div>").append(htmlEscape(message)).append("</div>");
  		}
  		if (trace != null) {
  			builder.append("<div style='white-space:pre-wrap;'>")
  					.append(htmlEscape(trace)).append("</div>");
  		}
  		builder.append("</body></html>");
  		return responseBody.syncBody(builder.toString());
  	}
docs: Render a default HTML "Whitelabel Error Page".
  <p>
  Useful when no other error view is available in the application.
  @param responseBody the error response being built
  @param error the error data as a map
  @return a Publisher of the {@link ServerResponse}