Changed failure check for a call to os::system().
Changed failure check for a call to os::system().

The previous check for `None()` was missing the case where the
program exited with a non-zero exit status.

Review: https://reviews.apache.org/r/66776/

kaysoky/mesos,jpeach/mesos,dforsyth/mesos,andschwa/mesos,reneploetz/mesos,andschwa/mesos,chhsia0/mesos,dforsyth/mesos,chhsia0/mesos,jpeach/mesos,andschwa/mesos,Gilbert88/mesos,reneploetz/mesos,dforsyth/mesos,abudnik/mesos,Gilbert88/mesos,jpeach/mesos,gsantovena/mesos,jpeach/mesos,chhsia0/mesos,chhsia0/mesos,abudnik/mesos,jpeach/mesos,gsantovena/mesos,Gilbert88/mesos,abudnik/mesos,kaysoky/mesos,kaysoky/mesos,gsantovena/mesos,reneploetz/mesos,Gilbert88/mesos,dforsyth/mesos,jpeach/mesos,andschwa/mesos,gsantovena/mesos,dforsyth/mesos,kaysoky/mesos,kaysoky/mesos,Gilbert88/mesos,andschwa/mesos,chhsia0/mesos,dforsyth/mesos,abudnik/mesos,chhsia0/mesos,andschwa/mesos,kaysoky/mesos,reneploetz/mesos,andschwa/mesos,gsantovena/mesos,reneploetz/mesos,Gilbert88/mesos,gsantovena/mesos,abudnik/mesos,abudnik/mesos,Gilbert88/mesos,jpeach/mesos,abudnik/mesos,dforsyth/mesos,gsantovena/mesos,reneploetz/mesos,kaysoky/mesos,reneploetz/mesos,chhsia0/mesos