patch 9.0.1523: some error messages are not marked for translation

Problem:    Some error messages are not marked for translation.
Solution:   Surround the messages in _(). (closes #12356)
diff --git a/src/popupmenu.c b/src/popupmenu.c
index 9cba669..7210420 100644
--- a/src/popupmenu.c
+++ b/src/popupmenu.c
@@ -1526,7 +1526,7 @@
     // pum_size being zero.
     if (pum_size <= 0)
     {
-	emsg(e_menu_only_exists_in_another_mode);
+	emsg(_(e_menu_only_exists_in_another_mode));
 	return;
     }