title: Correct way of throwing exceptions with Reactor
body: <p>I'm new to project <a href="https://projectreactor.io/" rel="noreferrer">Reactor</a> and reactive programming in general.</p>
  
  <p>I'm currently working on a piece of code similar to this:</p>
  
  <pre><code>Mono.just(userId)
      .map(repo::findById)
      .map(user-&gt; {
          if(user == null){
              throw new UserNotFoundException();
          }
          return user;
      })
      // ... other mappings
  </code></pre>
  
  <p>This example is probably silly and there are surely better ways of implementing this case, but the point is: </p>
  
  <p>Is it wrong to use a <code>throw new</code> exception in a <code>map</code> block or should I replace this with a <code>return Mono.error(new UserNotFoundException())</code>?</p>
  
  <p>Is there any actual difference in these two ways of doing?</p>