Notify superadmins if a Task fails

[BradHN1]

User Story.
As a superadmion
I want to know when a Task fails
So that I can investigate it and get it fixed.

Additional details/context
Issue #874 is caused by a nudge which started failing when the weekly Cadmin Nudge task encountered an exception. As the screenshot shows, the task was marked properly as Failed, but there was no notification of superadmins.

Notification should happen both through a Slack message to super_admin_alerts channel, and an email to the superadmin who created the task (or if that is difficult, can go to all superadmins for the deployed branch).

Value added.
This will save us some embarrassment since we can fix the problem more quickly.

Acceptance criteria.
This can be tested with a task that purposely encounters an exception, which can be run whenever we do a deployment.