ox: Small improvement to scrolling

* lisp/ox.el (org-export--dispatch-action): Small improvement to line
  by line scrolling.

lisp/ox.el

@@ -5509,8 +5509,14 @@ options as CDR."
 		(not expertp)
 		(memq key '(? ? ?\s ?\d)))
       (case key
-	(? (ignore-errors (scroll-up-line)))
-	(? (ignore-errors (scroll-down-line)))
+	(? (if (not (pos-visible-in-window-p (point-max)))
+		 (ignore-errors (scroll-up-line))
+	       (message "End of buffer")
+	       (sit-for 1)))
+	(? (if (not (pos-visible-in-window-p (point-min)))
+		 (ignore-errors (scroll-down-line))
+	       (message "Beginning of buffer")
+	       (sit-for 1)))
 	(?\s (if (not (pos-visible-in-window-p (point-max)))
 		 (scroll-up nil)
 	       (message "End of buffer")