Jenkinsfile: Partial deployment, follow up.

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

Jenkinsfile

@@ -186,7 +186,12 @@ See https://wiki.savoirfairelinux.com/wiki/Jenkins.jami.net#Configuration"
             steps {
                 script {
                     TARGETS.each { target ->
-                        unstash target
+                        try {
+                            unstash target
+                        } catch (err) {
+                            echo "Failed to unstash ${target}, skipping..."
+                            return
+                        }
                         def distribution = target - ~/^package-/
                         echo "Deploying packages for ${distribution}..."
                         sh """scripts/deploy-packages.sh \