Re-enable Jenkins notifications on ISO build/test failure

Originally created by @intrigeri on #11355 (Redmine)

Re-enable them one by one, carefully evaluating the outcome at each step, to make sure we continuously deliver a good enough UX to contributors.

Feature Branch: puppet-tails:feature/11355-reenable-jenkins-notifications;jenkins-jobs:feature/11355-reenable-jenkins-notifications

Edited May 15, 2020 by intrigeri

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