org-agenda.el (org-agenda-error): Use `user-error'

* org-agenda.el (org-agenda-error): Use `user-error'.

lisp/org-agenda.el

@@ -8730,7 +8730,7 @@ docstring of `org-agenda-show-1'."
 
 (defun org-agenda-error ()
   "Throw an error when a command is not allowed in the agenda."
-  (error "Command not allowed in this line"))
+  (user-error "Command not allowed in this line"))
 
 (defun org-agenda-tree-to-indirect-buffer (arg)
   "Show the subtree corresponding to the current entry in an indirect buffer.