This is inspired by the circuit breaker pattern used for distributed
systems. If too many requests fails, then it is better to immediately
fail new requests for a limited time to give the backend time to
recover.
By only forgetting a file in the cache at most once, we can ensure that
a broken file is only retrieved once again from the backend. If the file
stored there is broken, previously it would be cached and deleted
continuously. Now, it is retrieved only once again, all later requests
just use the cached copy and either succeed or fail immediately.
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