title: iOS FBAudienceNetwork Error codes consts
body: <p>In android when I get error I get an com.facebook.ads.AdError class.
  For that i can check which kind of error as occurred (like NETWORK_ERROR_CODE,NO_FILL_ERROR_CODE,...)</p>
  
  <p>In iOS i am getting a general NSError object in case of error, for example:</p>
  
  <p><code>  - (void)adView:(FBAdView *)adView didFailWithError:(NSError *)error; 
  </code></p>
  
  <p>how can I check which kind of error occurred in iOS? </p>