Middleware can use this to restart the fetch process.

Returning a Halt error in a middleware will stop the middleware execution chain.

Save dead jobs for 180 days, after that they will be purged.

Jobs will be reserved for 30 minutes by default.

We can't pass a nil across the Fetcher interface boundary so we'll use this sentinel value to mean nil.

A KnownError is one that returns a specific error code to the client such that it can be handled explicitly.