ox.el: Replace error with user-error to exit org-export-dispatch

* lisp/ox.el (org-export--dispatch-action): Replace error with
user-error a user-error so that user can quit `org-export-dispatch'
without entering debugging mode.

Modified from a patch proposal by Takaaki Ishikawa.
<https://lists.gnu.org/archive/html/emacs-orgmode/2019-12/msg00071.html>

lisp/ox.el

@@ -6929,7 +6929,7 @@ options as CDR."
       (org-export--dispatch-ui options first-key expertp))
      ;; q key at first level aborts export.  At second level, cancel
      ;; first key instead.
-     ((eq key ?q) (if (not first-key) (error "Export aborted")
+     ((eq key ?q) (if (not first-key) (user-error "Export aborted")
 		    (org-export--dispatch-ui options nil expertp)))
      ;; Help key: Switch back to standard interface if expert UI was
      ;; active.