`ValidationError` is now an `NSObject` subclass so that `ValidationDelegate` can declare an argument of type `UITextField:ValidationError`. When `ValidationError` is a Swift only class we get a compile error (in Xcode Version 7.0 GM (7A218)):
> Validator.swift:14:10: Method cannot be a member of an @objc protocol because the type of the parameter cannot be represented in Objective-C
Conflicts:
Validator/ValidationError.swift
9b34d138b7