patch 8.2.4012: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move the last error messages to errors.h.
diff --git a/src/undo.c b/src/undo.c
index 4c02551..ec972f0 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -464,7 +464,7 @@
 	{
 	    // This happens when the FileChangedRO autocommand changes the
 	    // file in a way it becomes shorter.
-	    emsg(_("E881: Line count changed unexpectedly"));
+	    emsg(_(e_line_count_changed_unexpectedly));
 	    return FAIL;
 	}
     }