Best way to handle errors from async closures in Swift 2?

there are many ways you can solve this, but i would recommend using a completion block which expects a Result Enum. this would probably be the most 'Swift' way.

the result enum has exactly two states, success and error, which a big advantage to the usual two optional return values (data and error) which lead to 4 possible states.

enum Result<T> {
    case Success(T)
    case Error(String, Int)
}

Using the result enum in a completion block finishes the puzzle.

let InvalidURLCode = 999
let NoDataCode = 998
func getFrom(urlString: String, completion:Result<NSData> -> Void) {
    // make sure the URL is valid, if not return custom error
    guard let url = NSURL(string: urlString) else { return completion(.Error("Invalid URL", InvalidURLCode)) }

    let request = NSURLRequest(URL: url)
    NSURLSession.sharedSession().dataTaskWithRequest(request) { data, response, error in
       // if error returned, extract message and code then pass as Result enum
        guard error == nil else { return completion(.Error(error!.localizedDescription, error!.code)) }

        // if no data is returned, return custom error
        guard let data = data else { return completion(.Error("No data returned", NoDataCode)) }

        // return success
        completion(.Success(data))
    }.resume()
}

because the return value is a enum, you should switch off of it.

getFrom("http://www.google.com") { result in
    switch result {
    case .Success(let data):
        // handle successful data response here
        let responseString = String(data:data, encoding: NSASCIIStringEncoding)
        print("got data: \(responseString)");
    case .Error(let msg, let code):
        // handle error here
        print("Error [\(code)]: \(msg)")
    }
}

another solution would be to pass two completion blocks, one for success and one for error. something along the lines of:

func getFrom(urlString: String, successHandler:NSData -> Void, errorHandler:(String, Int) -> Void)