tool_operate: don't discard failed parallel transfer result

tool_operate: don't discard failed parallel transfer result

- Save a parallel transfer's result code only when it fails and the
  transfer is not being retried.

Prior to this change the result code was always set which meant that a
failed result could be erroneously discarded if a different transfer
later had a successful result (CURLE_OK).

Before:

> curl --fail -Z https://httpbin.org/status/404 https://httpbin.org/delay/10
> echo %ERRORLEVEL%
0

After:

> curl --fail -Z https://httpbin.org/status/404 https://httpbin.org/delay/10
> echo %ERRORLEVEL%
22

Closes #xxxx