FallbackModel support

[sydney-runkle]

This PR adds a FallbackModel to the models folder that allows for delegation of a task to a new model if the current model fails with a 4xx or 5xx response. See the docs for a detailed example.

Easiest to review by toggling the "hide whitespace" button.

Fix #516

This is one of the items on our roadmap as well.

In a follow up PR:

• Rename ModelStatusError and rethink our error structure internally (before the next release)
• Consider errors with no status code (connection errors)

[niebloomj]

I love this

[dmontagu]

This looks great. Let's confirm the new exception names with Samuel tomorrow morning then merge.