title: String insted of whitelabel error page in webflux?
body: <p>When I use <strong><em>org.springframework.boot:spring-boot-starter-web</em></strong>, I can remove Whitelabel error page and instead of it to show my message, when to link not found resource:</p>
  
  <p><strong>application.yml</strong></p>
  
  <pre><code>server:
    error:
      whitelabel:
        enabled: false
  spring:
    mvc:
      throw-exception-if-no-handler-found: true
    resources:
      add-mappings: false
  </code></pre>
  
  <p><strong>NotFoundResource.class</strong></p>
  
  <pre><code>@RestControllerAdvice
  class NotFoundResource {
  
      @ExceptionHandler(NoHandlerFoundException.class)
      public String handleNotFoundResource() {
  
          return "Requested resource wasn't found on the server"
      }
  }
  </code></pre>
  
  <p>How do I do it with webflux? In weblux doesn't contain NoHandlerFoundException.class.</p>