sample of how to change AFHTTPSessionManager : GET response failure block updated to extract the error message sent from server and use inside an NSError object or however you might use it.

afnetworking_error_to_json

failure:^(NSURLSessionDataTask *task, NSError *error) {
                                       
     //convert error.data back to json
     NSError* jsonerror;
     NSDictionary* json = [NSJSONSerialization
                           JSONObjectWithData:error.userInfo[@"com.alamofire.serialization.response.error.data"] //1
                           options:kNilOptions 
                           error:&jsonerror];
     NSLog(@"json response data %@",json);
     
     
     //make a copy of error.userInfo to allow us to change just the fields we want updated.
     NSMutableDictionary *userInfo = [ error.userInfo mutableCopy];
     
     // grab the json error from the server.
     userInfo[NSLocalizedDescriptionKey]=json[@"error"][@"message"];
     
     // create a new NSError using most of what the original error contained but replacing our error message.
     NSError *betterError = [NSError errorWithDomain:error.domain code:error.code userInfo:userInfo];
     
     NSLog(@"Error %@",error);
     NSLog(@"Better Error %@",betterError);
     dispatch_async(dispatch_get_main_queue(), ^{
         completion(nil, betterError);
     });
 }];