It's useful to be able to know the exit code of the process when it
fails with a non-zero exit code.
To get this to work, I had to fix a bug in the implementation of
check_output. Previously, check_output would call both `p.communicate()`
and `p.poll()`. The former has the effect of waiting for EOF on the
input and then waiting for the child to exit, reaping it with
`waitpid(2)`. Unfortunately, `p.poll()` was hoping to be able to also
use `waitpid(2)` to retrieve the exit code of the process. But since the
child had already been reaped, the given pid no longer existed, and thus
`waitpid(2)` would return `ECHILD`.
Luckily the call to `p.poll()` is unnecessary, as the process already
provides `p.retcode()` for retrieving the exit code.
