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/insexpand.c b/src/insexpand.c
index 0978609..5dbd782 100644
--- a/src/insexpand.c
+++ b/src/insexpand.c
@@ -2864,7 +2864,7 @@
 
     if ((State & INSERT) == 0)
     {
-	emsg(_("E785: complete() can only be used in Insert mode"));
+	emsg(_(e_complete_can_only_be_used_in_insert_mode));
 	return;
     }