Fix error handler when omnibus builds fail
Fix error handler when omnibus builds fail

We were not calling `Mixlib::ShellOut` correctly and our error handler
for restoring the backup copy of delivery-cli when the omnibus build of
same fails was not succeeding. The result was that the build node was
left without a delivery-cli executable.

retep998/delivery-cli,chef/delivery-cli,retep998/delivery-cli,retep998/delivery-cli,retep998/delivery-cli,chef/delivery-cli,chef/delivery-cli