Node's `fetch` rejects every failure below HTTP as `fetch failed` and puts
the reason on `cause`. `describeError` in `src/errors.ts` gives the message
followed by the messages on the cause chain, and for an `AggregateError`,
which Node gives when every address of a host fails, the message of each
address. `run` prints it, and the backup logs and records it. `classify`
reads that text and calls a failure transient when `hasTransportCode`, now
exported by `src/retry.ts`, finds a transport errno on the chain. A failed
ML data fetch keeps its error as the cause. The bounded walk along the
chain moved from `src/retry.ts` to `src/errors.ts`.
A truncated download's message leaves the authentication failure to its
cause.
Model: opus-5-5
sneak