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.
Simplify Save As code & add UI for IO errors.
Plus some smaller cleanups (not altering functionality) in doSave() and
handleFileAddToWorkingSet(), and docs improvements.
Merge pull request #4499 from adobe/pflynn/saveas-cleanup

Simplify Save As & related code, improve docs (#4420)