Handle user canceled "error" in demo app

[mokagio]

While working on #761, I realized the demo app would crash when canceling ("Cancel" button) the web login.

This fixes it.

Testing

I verified this by running the app and pressing the "Cancel" button 😅

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• I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[mokagio]

I you ask Xcode, it will indent this as:

            } catch let error as NSError where
                        error.domain == ASWebAuthenticationSessionError.errorDomain
                        && error.code == ASWebAuthenticationSessionError.Code.canceledLogin.rawValue {

That doesn't make any sense to me, so I updated it manually.

[crazytonyli]