Mac:
- Better error messages
- The alert displayed contains an additional button to open the theme's folder (when clicked it will not dismiss the alert).
- `try` added where applicable to ArticleTheme inits
- `ArticleThemePlist` has fixed spelling of theme identifier and conforms to Equatable
- `ArticleTheme` now uses `ArticleThemePlist`
- `ArticleThemeDownloader` is now a class
- `ArticleThemeDownloader` will now download themes to Application Support/NetNewsWire/Downloads on macOS and iOS.
- `ArticleThemeDownloader` will remove downloaded themes from the Download folder when the application is closed.
- macOS app delegate now observes for theme download fails
- Error display code moved from SceneDelegate to SceneCoordinator so that it can use existing presentError on rootVC.
Stuart Breckenridge