title: NsError and userinfo
body: <p>I am building an IOS SDK.</p>
  
  <p>Basically, I am making an asynchronous network request (API Call). When there is an error, a dictionary is returned with keys "error_key", "error_description", "error_code".</p>
  
  <p>How can I represent those three information using NSError?</p>
  
  <p>I am doing the following:</p>
  
  <pre><code> NSMutableDictionary* details = [NSMutableDictionary dictionary];
  [details setValue: error_description forKey: NSLocalizedDescriptionKey];
  [NSError errorWithDomain: APPErrorDomain code: error_code userInfo: details];
  </code></pre>
  
  <p>How do I change the userinfo to also take into account error_key?</p>