Example of a completion handler that can throw

gistfile1.swift

func divide(numerator: Int, denominator: Int, completion: (() throws -> Int ) -> Void ) {
    
    if numerator == 0 || denominator == 0 {
        completion {
            throw NSError(domain: "Test", code: 10, userInfo: nil)
        }
    } else {
        completion {
            return enumerator / denominator
        }
    }
}

divide(100, denominator: 0) {
    do {
        let result = try $0()
        print(result)
    }
    catch {
        print("Error")
    }