patch 8.2.4008: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/ex_getln.c b/src/ex_getln.c
index 636dd38..01bd3b4 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -2664,7 +2664,7 @@
 {
     if (curbuf_lock > 0)
     {
-	emsg(_("E788: Not allowed to edit another buffer now"));
+	emsg(_(e_not_allowed_to_edit_another_buffer_now));
 	return TRUE;
     }
     return allbuf_locked();