patch 8.2.3987: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/digraph.c b/src/digraph.c
index a9fcba5..391b64a 100644
--- a/src/digraph.c
+++ b/src/digraph.c
@@ -2596,7 +2596,7 @@
 	    if (source_runtime(buf, 0) == FAIL)
 	    {
 		vim_free(buf);
-		return N_("E544: Keymap file not found");
+		return N_(e_keymap_file_not_found);
 	    }
 	}
 	vim_free(buf);