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`, so quak printed and recorded only that, and
`failures.json` called the failure `unknown`. `describeError` in
`src/errors.ts` gives the message followed by the messages on the cause
chain; `run` prints it, and the backup logs and records it. `classify`
calls a failure transient when `hasTransportCode`, now exported by
`src/retry.ts`, finds a transport errno on the chain. The bounded walk
along the chain moved from `src/retry.ts` to `src/errors.ts`, so both
use one walk.

Every message the backup logs or records, `imageMetadataError` and
`mlDataError` included, now carries its causes too.

Model: opus-5-5

README.md

@@ -134,6 +134,13 @@ 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. Node's `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`. A file whose download
+failed that way is in `failures.json` with that error, as a transient 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
@@ -347,7 +354,8 @@ 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