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

src/download/index.ts

@@ -135,15 +135,17 @@ const streamDecrypt = async (
                     // cannot tell a partial chunk from a corrupt one, so this
                     // is reported as the truncation it almost always is, with
                     // the authentication failure kept as the error's cause.
-                    // Only the pull is guarded: a sink failure on a chunk
-                    // that did authenticate is a disk error, not a
+                    // The message leaves that failure to the cause, which
+                    // `describeError` appends to what `quak backup` prints and
+                    // records. Only the pull is guarded: a sink failure on a
+                    // chunk that did authenticate is a disk error, not a
                     // truncation.
                     let pulled;
                     try {
                         pulled = pullStreamChunk(state, buffer);
                     } catch (err) {
                         throw new TruncatedStreamError(
-                            `download: stream truncated: response body ended with ${buffer.length} trailing bytes that did not authenticate as a final chunk (transfer stopped mid-chunk, or the data is corrupt)`,
+                            `download: stream truncated: response body ended with ${buffer.length} trailing bytes (transfer stopped mid-chunk, or the data is corrupt)`,
                             { cause: err },
                         );
                     }