Get alerted the moment a scheduled backup fails

A site owner describes their process like this: "When our nightly site backup fails,
I want an email sent to me and the developer immediately, so we can fix it before
another day passes without a good backup."

AVAILABLE TOOLS: [Backup plugin webhook (trigger), Uncanny Automator, email/SMTP,
Slack (optional escalation)]

Turn this into an implementation-ready automation recipe, thinking through what the
plain description leaves ambiguous:
1. Trigger: the exact failure event to listen for, and critically, what happens when
   the backup job itself never runs at all (a cron failure or hosting outage), since
   that produces no "failure" event to trigger on, only silence. Recommend a
   dead-man's-switch check that alerts if no success or failure event has fired
   within the expected window.
2. Conditions: distinguish a hard failure (backup didn't complete) from a partial
   success (database backed up but media files skipped due to a timeout), since the
   plain description treats these the same but they need different urgency.
3. Actions in order: send the immediate email with the specific error message from
   the backup plugin log, and if this is the second consecutive failure, escalate to
   Slack in addition to email since email alone clearly isn't getting acted on.
4. Failure handling: if the alerting email itself fails to send (SMTP misconfigured),
   what's the fallback channel, since an alert system with no fallback is exactly the
   single point of failure this recipe exists to prevent.
5. One edge case specific to this exact workflow: a backup that "succeeds" according
   to the plugin but produces a 0-byte or corrupted file. Decide whether the recipe
   should include a basic file-size sanity check rather than trusting the plugin's
   own success flag blindly.

Replace the workflow description and available tools with your own.