Fix addBodyClass on notFound page

[lcampanis]

Since the "not found" page isn't a registered route, there is no way of getting the routeName or theme for that route and the function breaks when the Not Found page is loaded.

addBodyClass should add an app-not-found class to the body when loading that view.

Note: There is still no clear way of how to get the current route theme though, since there is no route declared for not found. There should be a declared notFound route with a theme assigned or a global DEFAULT_THEME (food for thought)

[mikemurray]

Merged changes into 0.19.0