Error Handling in Swift 3

One way you can do is throwing your own errors on finding nil.

With having this sort of your own error:

enum MyError: Error {
    case FoundNil(String)
}

You can write something like this:

    do{
        let xmlString = try String(contentsOf: accessURL, encoding: String.Encoding.utf8)
        guard let xmlDict = XMLDictionaryParser.sharedInstance().dictionary(with: xmlString) else {
            throw MyError.FoundNil("xmlDict")
        }
        guard let models = xmlDict["Cygnet"] as? NSArray else {
            throw MyError.FoundNil("models")
        }
        self.cygnets = models
    } catch {
        print("error getting xml string: \(error)")
    }