title: collect all errors from a Flux<Void>
body: <p>I have a <code>Flux&lt;Message&gt;</code>, and a <code>Mono&lt;Void&gt; process(Message m)</code> function. The <code>process</code> function can return an error or nothing. I want to call the <code>process</code> function in parallel, and collect all the messageId that has an process error into <code>List&lt;String&gt;</code>.</p>
  <pre><code>@Data
  class Message {
      String messageId;
      String content;    
  }
  
  Flux.just(new Message(&quot;A&quot;, &quot;1&quot;), new Message(&quot;B&quot;, &quot;2&quot;))
      .flatMap(m -&gt; 
          process(m).onErrorReturn(m.messageId)
      )
  </code></pre>
  <p>But the problem is I can't return a type other than <code>Void</code> in the <code>onErrorXXX</code> methods.</p>