title: Conditional exception raising in a flow block
body: <p>While using Akka's data-flow DSL, I have twice encountered a need to throw an exception inside future, conditionally. This is how I am doing it:</p>
  
  <pre><code>flow {
    // ...
    if (someCond)
      shiftUnit(throw new SomeException)
    else
      Future().apply()
    // ...
  }
  </code></pre>
  
  <p>Is this the correct way to do it? Or is there a better approach?</p>