process: pipe() can raise exception

Previously the failed start of a service was not detected n el6, because
the fallback of check_output - pipe() - didn't throw exceptions on
failed starts. This is now fixed, pipe() is throwing (if asked)
exceptions to signla if th ecmd failed (the service didi not start).

Change-Id: I588d1b97d31553a1aaf05c0cd0462b118369179d
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=991375
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>