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

test/download/download.test.ts

@@ -74,7 +74,11 @@ import {
 } from "vitest";
 import { init, toBase64, STREAM_CHUNK_SIZE } from "../../src/crypto/index.js";
 import { ApiClient } from "../../src/api/client.js";
-import { ApiError, TruncatedStreamError } from "../../src/errors.js";
+import {
+    ApiError,
+    describeError,
+    TruncatedStreamError,
+} from "../../src/errors.js";
 import type { RetryOptions } from "../../src/retry.js";
 import {
     downloadFile,
@@ -834,6 +838,11 @@ describe.each(entryPoints)(
             expect(((err as Error).cause as Error).message).toMatch(
                 /authentication failed/i,
             );
+            // As quak prints it and records it in `failures.json`, the error
+            // states the authentication failure once, from its cause.
+            expect(describeError(err)).toMatch(
+                /^download: stream truncated: response body ended with \d+ trailing bytes \(transfer stopped mid-chunk, or the data is corrupt\): secretstream chunk authentication failed$/,
+            );
 
             // And, as with every other failure, the destination is untouched
             // and no staged temp file survives.