An extension to get serialized error data when server sends some data on the error response.

NSError+AFNetworking.swift

extension NSError {
    var serializedErrorData: [String : AnyObject]? {
        get {
            if let responseData = userInfo[AFNetworkingOperationFailingURLResponseDataErrorKey] as? NSData {
                if let data = try? NSJSONSerialization.JSONObjectWithData(responseData, options: NSJSONReadingOptions.AllowFragments) as? [String : AnyObject] {
                    return data
                }
            }
            return nil
        }
    }
}