title: webflux Mono<T> onErrorReturn not called
body: <p>this is my HandlerFunction</p>
  
  <pre><code>    public Mono&lt;ServerResponse&gt; getTime(ServerRequest serverRequest) {
      return time(serverRequest).onErrorReturn("some errors has happened !").flatMap(s -&gt; {
          // this  didn't called
          return ServerResponse.ok().contentType(MediaType.TEXT_PLAIN).syncBody(s);
      });
  }
  </code></pre>
  
  <p>time(ServerRequest serverRequest) method is</p>
  
  <pre><code>private Mono&lt;String&gt; time(ServerRequest request) {
      String format = DateTimeFormatter.ofPattern("HH:mm:ss").format(LocalDateTime.now());
      return Mono.just("time is:" + format + "," + request.queryParam("name").get());
  }
  </code></pre>
  
  <p>when i don't using param "name",it will throw one NoSuchElementException;
  But, the Mono onErrorReturn not working!
  why or what do i wrong?</p>