user Log In, let the users in without errors (SWIFT) (Parse) - swift

Question

i'm making an app that required Logging In. the problem is when i run my app to try it and type wrong user info it proceed to the next view controller without giving the error !. Heres my code i don't whats the problem !
{
#IBOutlet weak var ActivityIndicator: UIActivityIndicatorView!
#IBOutlet weak var Message: UILabel!
#IBOutlet weak var UsernameTextField: UITextField!
#IBOutlet weak var PasswordTextField: UITextField!
override func viewDidLoad() {
super.viewDidLoad()
ActivityIndicator.hidden = true
ActivityIndicator.hidesWhenStopped = true
// Do any additional setup after loading the view.
}
#IBAction func LogInButtonTapped(sender: AnyObject) {
LogIn()
}
func LogIn() {
// Start activity indicator
ActivityIndicator.hidden = false
ActivityIndicator.startAnimating()
// if there is a user
var user = PFUser()
user.username = UsernameTextField.text
user.password = PasswordTextField.text
PFUser.logInWithUsernameInBackground(UsernameTextField.text, password:PasswordTextField.text) {
(user: PFUser?, error: NSError?) -> Void in
if user != nil {
dispatch_async(dispatch_get_main_queue()) {
self.performSegueWithIdentifier("LogInToHomeVC", sender: self)}
println("Logged In")
} else {
self.ActivityIndicator.stopAnimating()
if let Message: AnyObject = error!.userInfo!["error"] {
self.Message.text = "\(Message)"}
println("Could Not Find User")
}
}
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}
so the question is how to let the user try again and not let him enter the Home Page?