Fix bug #4480 (Save/Close All wrt canceled untitled saves) & add more unit tests

Fix bug #4480 (Save All & Close All act oddly when save canceled on untitled document) - stop the save-all chain immedately when a save fails due to user choice.

Add untitled-document unit tests for Save All / Close All cases (including #4480), and add more checks to existing untitled-document tests.

Also clean up DocumentCommandHandlers docs and improve some assertions.