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

README.md

@@ -141,6 +141,18 @@ matters.
 | `2`  | Another backup of the same directory is running. This run sent no request and changed nothing.                                                                                    |
 | `3`  | There is no usable session: none is saved, the saved one is corrupt, or the server no longer accepts it. Run `quak login` as the user the cron job runs as.                       |
 
+`<message>` is the error's message, followed by the message of each error that
+caused it. Under Node, as `yarn quak` runs quak, `fetch` reports every failure
+below HTTP as just `fetch failed` and gives the reason as its cause, so a run
+that could not reach the server prints, for example,
+`quak: fetch failed: connect ECONNREFUSED 1.2.3.4:443`. When the server has
+several addresses and none of them could be reached, the line gives the reason
+for each, separated by `, `. A file whose download failed that way is in
+`failures.json` with that error, as a transient failure. So is each file with no
+ML data cached when the ML data fetch failed that way, its error starting
+`ML data: `. The `~/bin/quak` that `make install` builds runs under bun, and
+prints bun's own wording for such a failure.
+
 After a `1`, the next run fetches each missing original again, fetches the ML
 data that is not cached, and rebuilds the album links. An album quak could not
 decrypt, or a file in `failures.json` because quak could not decrypt it, keeps
@@ -354,7 +366,8 @@ quak/
         exif.ts         EXIF read from an image's bytes with exifreader
         mldata-fetch.ts fetch + decrypt per-file ML data
         filename.ts     safe file names from server metadata
-        errors.ts       error types shared across layers
+        errors.ts       error types shared across layers, and how an error
+                        is described
         retry.ts        retry classifier + exponential backoff with jitter
         thumbnails.ts   detect + regenerate missing thumbnails
         client.ts       high-level Client class assembled from the above