Better fallback on Edge Function failures

### Description

We are reliant on edge functions for A/B testing, but they are proving annoyingly unreliable on Netlify. For the last support ticket I raised, I was pointed in the direction of:

https://docs.netlify.com/edge-functions/optional-configuration/#configure-error-handling-for-an-edge-function

Which is something we should look to implement in order to fallback/bypass edge functions in the case where they're down. It should be ensured that the fallback does work correctly, given that we render HTML content inside an edge function, we need _some_ of that to render upon failure (the `control` content)

### Epic/Story

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate