Beta Plugin: Safe fallback if Beta fatals

Since the Beta plugin lives side-by-side with the Jetpack plugin like this:
- jetpack-dev
- jetpack

The beta plugin should be able to fallback gracefully to the "stable" version of Jetpack if it encounters any fatals.

Basically, deactivate itself and for bonus points send an email.

The issue names no files, tests, or entry points. Start by locating how the Beta plugin activates, handles fatal errors, and coexists with the stable Jetpack plugin; define how self-deactivation and optional email notification should be verified, including recovery when a fatal occurs.

Written by the indexing model from the issue text.