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/undo.c b/src/undo.c
index a376150..40f97b1 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -3180,7 +3180,7 @@
 	return;		    // nothing changed before
     if (curbuf->b_u_curhead != NULL)
     {
-	emsg(_("E790: undojoin is not allowed after undo"));
+	emsg(_(e_undojoin_is_not_allowed_after_undo));
 	return;
     }
     if (!curbuf->b_u_synced)