waitsendpay: indicate which channel direction the error was.

You can figure this yourself by knowing the route, but it's better to report
it directly here.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/pay.h

@@ -19,6 +19,7 @@ struct routing_failure {
 	enum onion_type failcode;
 	struct pubkey erring_node;
 	struct short_channel_id erring_channel;
+	int channel_dir;
 	u8 *channel_update;
 };