A network failure is printed and recorded with its reason (closes #196)

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

TODO.md

@@ -17,9 +17,11 @@ pre-1.0
 
 # Next Step
 
-Three bugs are open: https://git.eeqj.de/sneak/quak/issues/197,
+Five bugs are open: https://git.eeqj.de/sneak/quak/issues/197,
-https://git.eeqj.de/sneak/quak/issues/198 and
+https://git.eeqj.de/sneak/quak/issues/198,
-https://git.eeqj.de/sneak/quak/issues/199. The check of quak against a live Ente
+https://git.eeqj.de/sneak/quak/issues/199,
+https://git.eeqj.de/sneak/quak/issues/205 and
+https://git.eeqj.de/sneak/quak/issues/206. The check of quak against a live Ente
 account, https://git.eeqj.de/sneak/quak/issues/187, needs an account only sneak
 has, so it waits on him.