get_request_result: handle missing error code

[presto8]

The API response from ETrade does not always contain the error code, which will cause a KeyError exception. This changeset adds a guard to handle this condition.

  File "/home/avo/src/et/pyetrade_dev/order.py", line 644, in place_option_order
    return self.place_equity_order(**kwargs)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/home/avo/src/et/pyetrade_dev/order.py", line 669, in place_equity_order
    return self.perform_request(self.session.post, api_url, payload, "xml")
           ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/avo/src/et/pyetrade_dev/order.py", line 474, in perform_request
    return get_request_result(req, resp_format)
  File "/home/avo/src/et/pyetrade_dev/order.py", line 58, in get_request_result
    f'Etrade API Error - Code: {req_output["Error"]["code"]}, Msg: {req_output["Error"]["message"]}'
                                ~~~~~~~~~~~~~~~~~~~^^^^^^^^