title: Java Reactor : unhandled exception from map(), but is handled from a fromCallable()
body: <p>In the following two examples, the first one (with the <code>map()</code>) tells me that there's an unhandled exception from <code>readTree()</code>, but not in the second example. Why so ? Thx!</p>
  
  <pre><code>Mono.just(jsonString)
  .map(jsonString1 -&gt; {
      return new ObjectMapper().readTree(jsonString1);
  })
  .onErrorResume(IOException.class, error -&gt; {
      return Mono.error(new InvalidContentException("error with the JSON. " + error));
  })
  </code></pre>
  
  <pre><code>Mono.fromCallable(() -&gt; {
      return new ObjectMapper().readTree(jsonString);
  })
  .onErrorResume(IOException.class, error -&gt; {
      return Mono.error(new InvalidContentException("error with the JSON. " + error));
  })
  </code></pre>