[mms] Fix regression where auto-saved drafts were not being deleted when cancelling compose message in dynamic view.
Non-authenticated AJAX actions are only supported in handlers, not hooks
Hacky: for 'fallback' auth used with AJAX, need to store authentication exception in case non-auth access is not available

Otherwise, appInitFailure() is never triggered for app (i.e. IMP auto
draft save is not triggered).

For H6, need cleaner way to bootstrap an access and/or route all logout
events through a single object.  Right now, Horde_ErrorHandler catches
auth exceptions, but it does redirects only.  Should refactor so it
handles non-interactive accessess too.