C++ Template App Follow-Ups

[dacharyc]

• Handle auth errors
• Adjust button and window constraints so the button labels are less likely to disappear
• Use a delegate to have the HomeController change its own state, instead of letting DatabaseManager mutate its state, per this PR review comment: (DOCSP-36597): C++ Template App, MVC style #197 (comment)

Note for reviewer: after making this change, IntelliSense is saying the DatabaseManager constructor is never used, and is warning me that the _delegate I added to the DatabaseManager may be null. I'm not getting those warnings in the other places that have delegates (AuthManager/AppController, for example). So I think I may have missed something here - BUT it is all working properly.

[cbush]

Try from clean build. Not sure what the IntelliSense error is. The default (no args) constructor of DatabaseManager should be deleted by the fact that the other constructor exists, so there should be no way that constructor is unused.