Error handling in PromiseKit example

```swift
UIApplication.sharedApplication().networkActivityIndicatorVisible = true
            
when(getQuestions(session.id, date: date), getAnswers(session.id, date: date)).then {
    (questions: [SessionQuestion], answers: [SessionAnswer]) -> Void in
    self.sessionQuestions = questions
    self.sessionAnswers = answers
    
    self.tableView.reloadData()
    }.always {
        UIApplication.sharedApplication().networkActivityIndicatorVisible = false
    }
    .error({ (error: ErrorType?) -> Void in
        if let error = error as? PromiseKit.Error {
            switch error {
            case PromiseKit.Error.When(_, let errorType):
                switch errorType {
                case ServerError.Unauthorized:
                    print("ServerError.Unauthorized")
                    
                    if let navigationController = self.navigationController as? ClientNavigationController {
                        dispatch_async(dispatch_get_main_queue()) {
                            navigationController.logOut()
                        }
                    }
                default:
                    print("ServerError.Failure")
                }
            default :
                print("default")
            }
        }
    })
```