Pass a view controller when handing over errors to host apps

[ctarda]

In we PR #525 we added a couple of delegate methods to allow host apps to decide if they want to handle specific errors.

func handleError(_ error: Error, onCompletion: @escaping (UIViewController) -> Void) assumes that host apps will handle an error by providing custom UI back to WPAuthenticator, but we have found a case where the host app just want to present a FancyAlertController itself.

So we would need to pass a reference to the VC originating the error as well.

[ctarda]

🤔 this might not be necessary after all. Closing for now.