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/menu.c b/src/menu.c
index 88b8b84..614196a 100644
--- a/src/menu.c
+++ b/src/menu.c
@@ -512,7 +512,7 @@
 	if (*dname == NUL)
 	{
 	    // Only a mnemonic or accelerator is not valid.
-	    emsg(_("E792: Empty menu name"));
+	    emsg(_(e_empty_menu_name));
 	    goto erret;
 	}