Reverted cleanup step of `verify-reviews.py`.
Reverted cleanup step of `verify-reviews.py`.

Reverts the improvements made on the support script `verify-reviews.py`
in 3badf7179992e61f30f5a79da9d481dd451c7c2f. Sadly, these changes have
broken our Azure based Mesos CI as seen in MESOS-9514.

After comparing the code before and after the commit, we can see that
the line failing in CI, `HEAD = shell("git rev-parse HEAD")`, is
misplaced. It was previously located in the exit process in the
`cleanup()` function and it is now back there, surrounded by a
try/catch for error handling.

Review: https://reviews.apache.org/r/69697/

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