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/digraph.c b/src/digraph.c
index 391b64a..4c45e08 100644
--- a/src/digraph.c
+++ b/src/digraph.c
@@ -2661,7 +2661,7 @@
 		    || *kp->from == NUL || *kp->to == NUL)
 	    {
 		if (kp->to != NULL && *kp->to == NUL)
-		    emsg(_("E791: Empty keymap entry"));
+		    emsg(_(e_empty_keymap_entry));
 		vim_free(kp->from);
 		vim_free(kp->to);
 	    }