Investigate whether Google Play listings/what's new can be fetched in an upstream job

[JohanLorenzo]

Bug 1383642 caused 55.0b12 to not be uploaded. @Callek suggested to split the pushapk job into 2:

1. One download job which will fetch listings/what's new from store_l10n and upload them as artifacts.
2. The second job which will take the artifacts and pass them to mozapkpublisher.

Per @Callek and the TC docs, Taskcluster now allows a job to depend on failed jobs if requires: "all-resolved", in the task definition. This means, we can fail the first task in the event of stores_l10n not being online. mozapkpublisher will be called in a way that listings/what's new won't be updated (see mozilla-releng/mozapkpublisher#36, for instance)

We may need scriptworker to:

• support non-completed tasks
• to not download artifacts from non-completed tasks.
• to notify the script about missing artifacts

We'll also require to add a new type of task in the in-tree task graph.

cc @MihaiTabara @escapewindow

[JohanLorenzo]

This was done in https://bugzilla.mozilla.org/show_bug.cgi?id=1385401