Resumability works by asking the installer for its progress and for the appcast item we've given it before.
If an automatic silent download occurs, we abort it after the installer tool finishes with its basic steps.
Then, a user initiated or scheduled check can resume the update. The scheduled check fires during a longer-than-normal
span since usually the user can install by quitting the app, unless the app has been alive for a very long time.
One exception to this is when the update is marked critical; in that case, the scheduled check is fired immediately after
an automatic update download. Subsequent attempts will be fired using a normal update check interval.
Todo: delta updates probably don't work when they fail and fallback to a regular update
Zorg