title: How can I get more useful information from NSError?
body: <p>I want to get some useful information from <code>NSError</code>.
  If I print out <code>[error userInfo]</code>, I get the following:</p>
  
  <pre><code>{
      NSFilePath = "/Users/apple/Library/Application Support/iPhone Simulator/5.1/Applications/08260B6A-4D65-48DF-ADD1-FFC8750081E8/Documents/abc";
      NSUnderlyingError = "Error Domain=NSPOSIXErrorDomain Code=17 \"The operation couldn\U2019t be completed. File exists\"";
  }
  </code></pre>
  
  <p>I want to show the last line : "File exists", but how can i pick it out?</p>
  
  <p>I tried:</p>
  
  <pre><code>localizedDescription
  localizedFailureReason
  localizedRecoverySuggestion
  localizedRecoveryOptions
  recoveryAttempter
  </code></pre>
  
  <p>Non of them show "File exists".</p>