Notification mechanism if API call consistently failing

[mloughran]

Currently, as long as stale data is available, the only notice you'll get that an API is failing is a warning in the logs. However, if for example, if the API you're consuming has changed, you'd like to be notified during the stale period so that you can update your code.

I'm thinking webhooks. Maybe a global configurable address which if supplied it would be pinged on failure during the stale period.