title: Error handling with promises in Koa
body: <p>If I yield promises in Koa, they may be rejected: </p>
  
  <pre><code>function fetch = (){
    var deferred = q.defer(); 
    //Some async action which calls deferred.reject();
    return deferred.promise;
  }
  
  this.body = yield fetch(); //bad, not going to work
  </code></pre>
  
  <p>Is there a error-handling pattern in Koa to handle this besides explicitly unwrapping the promise with <code>then</code> and handling the error explicitly? </p>