When soup fails via the EXIT trap after it has begun modifying the system, print
a prominent UPGRADE INCOMPLETE banner instructing the user to run soup again to
resume and complete the update. Gated on a new SOUP_UPGRADE_STARTED flag set at
the start of the hotfix and upgrade branches, so pre-flight gate failures (ES
compatibility, disk, network) that abort before any changes are made do not show
it.
Mike Reeves
618712469e