paper_trading/alpaca.py: submitOrder response list never read after thread joins

[atharvajoshi01]

While fixing the threading bug in #1399 (merged in #1410), I noticed that the respSO lists passed to each submitOrder thread are never read after the threads are joined.

respSO = []
tSubmitOrder = threading.Thread(
    target=self.submitOrder,
    args=(qty, self.stockUniverse[index], "sell", respSO),
)
tSubmitOrder.start()
threads.append(tSubmitOrder)

After x.join(), the collected responses in respSO are ignored. If submitOrder is meant to return order status or fill information through this list, it's being silently discarded. If it's not needed, the parameter could be removed to simplify the code.

Spotted during the #1399 fix.

[atharvajoshi01]

Now that #1410 fixed the threading so submitOrder actually runs in threads, this becomes more relevant. The respSO lists are being populated correctly now but still discarded after join().

Simplest fix would be to log failed orders after the joins. Silent failures in paper trading defeat the purpose. I can put up a PR for this if maintainers want.