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, also for ML data that is not a
JSON object. The library gives the reasons from its last finished ML data
fetch 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`. `backup-metadata`
writes `mlDataError` for it and exits 1; `runMetadataBackup` returns
`filesWithMLDataError`.

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

test/cli/backup.test.ts

@@ -938,13 +938,14 @@ const ML_PAYLOAD = { face: { faces: [] }, clip: { embedding: [0.5, 0.25] } };
 
 class MLClient extends MockClient {
     mlError?: string;
-    async fetchMLData(args: {
-        fileIDs: number[];
-    }): Promise<Map<number, MLData>> {
+    async fetchMLData(args: { fileIDs: number[] }): Promise<{
+        payloads: Map<number, MLData>;
+        errors: Map<number, unknown>;
+    }> {
         if (this.mlError) throw new Error(this.mlError);
-        const result = new Map<number, MLData>();
-        if (args.fileIDs.includes(100)) result.set(100, ML_PAYLOAD);
-        return result;
+        const payloads = new Map<number, MLData>();
+        if (args.fileIDs.includes(100)) payloads.set(100, ML_PAYLOAD);
+        return { payloads, errors: new Map() };
     }
 }
 
@@ -1517,6 +1518,7 @@ describe("backup album folders", () => {
         contentConcurrency: 5,
         fetchMLData: async () => {},
         mlData: async () => undefined,
+        mlDataError: () => undefined,
         whoami: () => ({ email: "backup@example.com", userID: USER_ID }),
     });