[Twisted-Python] spawnProcess with timeout and same error handling on Unix and Windows

[Adi Roiban]

Hi,

If I call spawnProcess with a bad executable, on Unix the process will
exit with exit code 1 while on Windows an OSError is raised.

I am working on a multi-OS software and to make my life easier I ended
up with this hack inspired by _BackRelay

https://gist.github.com/adiroiban/bac493f00ce5e94738ce

----

Is there something already in Twisted doing this? Am I reinventing the wheel?

If not, do you think that it would help to update _BackRelay to
support timeout and cross-os error handling?


-- 
Adi Roiban