This is a follow up to commit 4ee93922, which introduced partial
deployments.

The unstash operation also needs to be handled specially, as it may
now fail attempting to unstash an item that could not be stashed
previously.

* Jenkinsfile ('Sign & deploy packages'): Skip target deployment when
it fails to be unstashed.

Change-Id: I59341e4c3bb007125402f4b28979d0e8e963757d