Made the default executor fail kills if the response isn't "200 OK".
Made the default executor fail kills if the response isn't "200 OK".

The default executor's `Future<Nothing> kill(const ContainerID&, int)`
method returns `Nothing()` if the agent responded to the
`KILL_NESTED_CONTAINER` call, regardless of the response.

This patch updates the method, so that it returns a failure if the
response is not "200 OK".

Review: https://reviews.apache.org/r/65693/

andschwa/mesos,gsantovena/mesos,dforsyth/mesos,jpeach/mesos,asamerh4/mesos,abudnik/mesos,jpeach/mesos,reneploetz/mesos,chhsia0/mesos,reneploetz/mesos,reneploetz/mesos,kaysoky/mesos,dforsyth/mesos,Gilbert88/mesos,reneploetz/mesos,chhsia0/mesos,andschwa/mesos,dforsyth/mesos,asamerh4/mesos,Gilbert88/mesos,abudnik/mesos,dforsyth/mesos,asamerh4/mesos,jpeach/mesos,asamerh4/mesos,abudnik/mesos,gsantovena/mesos,andschwa/mesos,dforsyth/mesos,dforsyth/mesos,dforsyth/mesos,jpeach/mesos,Gilbert88/mesos,abudnik/mesos,Gilbert88/mesos,andschwa/mesos,abudnik/mesos,asamerh4/mesos,Gilbert88/mesos,gsantovena/mesos,chhsia0/mesos,gsantovena/mesos,gsantovena/mesos,kaysoky/mesos,chhsia0/mesos,abudnik/mesos,andschwa/mesos,reneploetz/mesos,andschwa/mesos,abudnik/mesos,Gilbert88/mesos,asamerh4/mesos,andschwa/mesos,chhsia0/mesos,gsantovena/mesos,kaysoky/mesos,kaysoky/mesos,jpeach/mesos,jpeach/mesos,kaysoky/mesos,reneploetz/mesos,asamerh4/mesos,gsantovena/mesos,kaysoky/mesos,Gilbert88/mesos,kaysoky/mesos,jpeach/mesos,chhsia0/mesos,chhsia0/mesos,reneploetz/mesos