title: Custom error message json object with flask-restful
body: <p>It is easy to propagate error messages with flask-restful to the client with the <code>abort()</code> method, such as</p>
  
  <pre><code>abort(500, message="Fatal error: Pizza the Hutt was found dead earlier today
  in the back seat of his stretched limo. Evidently, the notorious gangster
  became locked in his car and ate himself to death.")
  </code></pre>
  
  <p>This will generate the following json output</p>
  
  <pre><code>{
    "message": "Fatal error: Pizza the Hutt was found dead earlier today
         in the back seat of his stretched limo. Evidently, the notorious gangster
         became locked in his car and ate himself to death.", 
    "status": 500
  }
  </code></pre>
  
  <p>Is there a way to customise the json output with additional members? For example:</p>
  
  <pre><code>{
    "sub_code": 42,
    "action": "redirect:#/Outer/Space"
    "message": "You idiots! These are not them! You've captured their stunt doubles!", 
    "status": 500
  }
  </code></pre>