Major Description
We use stale-if-error to return cached content to clients in case of origin server failure. However, if an object is removed from the cache by a PURGE request, and the origin fails before another client requests the same object, an error will be served.
Instead of using PURGE, we'd like a way to mark the cache item as stale, but not delete it, so it will be revalidated on the next request, and if the origin is unavailable, it can be served by stale-if-error.