ML data that does not decrypt is recorded with its reason (closes #198)

`fetchMLDataBatch` dropped an entry that did not decrypt, gunzip or parse,
so the file looked like one with no ML data and the run exited 0. It now
returns the reason beside the payloads. The library keeps the reasons from
its last ML data fetch in memory only and gives them in
`status().mlDataErrors` and the `fetchMLData` "done" event. `quak backup`
writes the reason as `mlDataError` for a file with no ML data cached and
records the file in `failures.json` as `ML data: REASON`. `backup-metadata`
writes `mlDataError` for it and exits 1; `runMetadataBackup` now returns how
many files have `mlDataError`.

Judgement call: a file whose ML data is cached keeps it in its JSON when a
newer copy does not decode, as when the fetch fails.

Model: opus-5-5

src/cli-commands.ts

@@ -394,11 +394,11 @@ export const backupMetadataCommand = async (
         // Refresh first so the dump holds current account state, not what the
         // cache last held; a failed refresh throws.
         await lib.fresh();
-        const { failedMLBatches } = await runMetadataBackup(lib, client, dir, {
+        const { mlDataErrors } = await runMetadataBackup(lib, client, dir, {
             exif: opts.exif || opts.all,
             onProgress: (msg) => ctx.stderr.write(msg + "\n"),
         });
-        return failedMLBatches > 0 ? 1 : 0;
+        return mlDataErrors > 0 ? 1 : 0;
     } finally {
         await lib.close();
     }