Improved the fetcher exit status log message.
Improved the fetcher exit status log message.

When the fetcher fails, we emit a message with its exit status,
but the `status` is the return value from wait(2) so we should
be using WSTRINGIFY.

Review: https://reviews.apache.org/r/63594/

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