walletrpc: explicitly note we discard utx on unsuccessful broadcast

If we can't broadcast the transaction, you're going to have to start
over regardless.  It's a tal child of cmd, which we free here.

wallet/walletrpc.c

@@ -68,7 +68,7 @@ static void wallet_withdrawal_broadcast(struct bitcoind *bitcoind UNUSED,
 		was_pending(command_success(cmd, response));
 	} else {
 		was_pending(command_fail(cmd, LIGHTNINGD,
-					 "Error broadcasting transaction: %s",
+					 "Error broadcasting transaction: %s. Unsent tx discarded",
 					 output));
 	}
 }