added check and tests to stop add-units earlier if the destination machine doesn't exist, lp:1384732
use errors annotate instead of fmt.Errorf
Merge pull request #969 from mattyw/add-unit-fails-too-late

add-unit needs to fail earlier if destination machine doesn't exist

Bug fix for https://bugs.launchpad.net/juju-core/+bug/1384732

juju add-unit my-service --to 3 fails too late if machine 3 doesn't exist.