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

@@ -17,8 +17,7 @@ pre-1.0
 
 # Next Step
 
-Four bugs are open: https://git.eeqj.de/sneak/quak/issues/196,
-https://git.eeqj.de/sneak/quak/issues/197,
+Three 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/199. The check of quak against a live Ente
 account, https://git.eeqj.de/sneak/quak/issues/187, needs an account only sneak
@@ -29,6 +28,18 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-15: A network failure is printed and recorded with its reason, not as
+  just `fetch failed` (issue 196). `describeError` in `src/errors.ts` gives an
+  error's message followed by the message of each error on its `cause` chain,
+  such as `fetch failed: connect ECONNREFUSED 1.2.3.4:443`, and each address's
+  reason when a host has several and none could be reached. `run` prints that,
+  and the backup logs it and records it in `failures.json`, where `classify` now
+  calls the failure transient when the chain holds an errno that
+  `hasTransportCode` in `src/retry.ts` recognises, for a failed ML data fetch
+  too. The walk along the chain moved from `src/retry.ts` to `src/errors.ts`,
+  which both use. Tests refuse every connection of a backup, then only one
+  file's download, and fail the ML data fetch.
+
 - 2026-10-15: The README says that `make install` needs nix (issue 200), in
   Getting Started and in "Running the backup from cron": `make build-bin` runs
   bun through `nix-shell -p bun`. The `Makefile` is unchanged.