Pypi mirror wrapper script should abort safely.

* modules/pypimirror/templates/run-mirror.sh.erb: Aborting the
mirror downloader prematurely should not allow the indexer to start
running. This has caused corrupt indexes in the past.

Change-Id: I42e3390910909e02c84f3a1d9641b4fc3b004d31
Reviewed-on: https://review.openstack.org/24021
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Paul Belanger <paul.belanger@polybeacon.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins