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,...)
In iOS i am getting a general NSError object in case of error, for example:
- (void)adView:(FBAdView *)adView didFailWithError:(NSError *)error;
There are no error domains or constants in FBAudienceNetwork iOS SDK.
com.facebook.ads.AdError.NO_FILL_ERROR_CODE is a local error code for FB SDK (android). It is not necessary that you can find those codes in iOS.
To see detail of error just log your error like so:
NSLog(@"Error %@",[error localizedDescription]);
I'll suggest you to find error domain first. iOS usage Domains and Codes for error identification. Reference
NSURLErrorDomain codes are here.