org-table.el: Fix message

* lisp/org-table.el (org-table-toggle-coordinate-overlays):
Fix the message sent when coordinates are not displayed.

lisp/org-table.el

@@ -3768,14 +3768,16 @@ FACE, when non-nil, for the highlight."
 (defun org-table-toggle-coordinate-overlays ()
   "Toggle the display of Row/Column numbers in tables."
   (interactive)
-  (setq org-table-overlay-coordinates (not org-table-overlay-coordinates))
-  (message "Tables Row/Column numbers display turned %s"
-	   (if org-table-overlay-coordinates "on" "off"))
-  (when (and (org-at-table-p) org-table-overlay-coordinates)
-    (org-table-align))
-  (unless org-table-overlay-coordinates
-    (mapc 'delete-overlay org-table-coordinate-overlays)
-    (setq org-table-coordinate-overlays nil)))
+  (if (not (org-at-table-p))
+      (user-error "Not on a table")
+    (setq org-table-overlay-coordinates (not org-table-overlay-coordinates))
+    (when (and (org-at-table-p) org-table-overlay-coordinates)
+      (org-table-align))
+    (unless org-table-overlay-coordinates
+      (mapc 'delete-overlay org-table-coordinate-overlays)
+      (setq org-table-coordinate-overlays nil))
+    (message "Tables Row/Column numbers display turned %s"
+	     (if org-table-overlay-coordinates "on" "off"))))
 
 ;;;###autoload
 (defun org-table-toggle-formula-debugger ()