Failed runs end with one fatal line; failure lines are errors

A command that exits 1 because files or albums failed now ends with one
fatal line saying how many failed and where to look, so -q never leaves
a non-zero exit unexplained: backup points to failures.json, backup-metadata
to mlDataError in each file's JSON, and fix-missing-thumbnails to its
Failed files list. The progress callbacks of the backup, the metadata
backup and the thumbnail helpers take a level, so every line that reports
a failure is logged at error. The library's operation names are shown in
plain words. The README says which lines are error and warning, and which
summary lines --json replaces.

Model: opus-5-5

test/thumbnails/thumbnails.test.ts

@@ -544,11 +544,22 @@ describe("listMissingThumbnails", () => {
         const client = await login(counted.fetch, { ...noWait });
         const lib = await openLib(client);
 
-        const missing = await listMissingThumbnails(lib, client);
+        const errors: string[] = [];
+        const missing = await listMissingThumbnails(
+            lib,
+            client,
+            (msg, level) => {
+                if (level === "error") errors.push(msg);
+            },
+        );
         lib.close();
 
-        // Only the genuinely empty thumbnail is reported.
+        // Only the genuinely empty thumbnail is reported. The file that
+        // could not be checked is reported as an error.
         expect(missing.map((m) => m.fileID)).toEqual([101]);
+        expect(errors).toEqual([
+            expect.stringMatching(/^\[.*\] Could not check file-102\.jpg: /),
+        ]);
         // And the 500 was retried rather than accepted as an answer: four
         // attempts is the library default.
         expect(counted.matched()).toBe(4);
@@ -699,7 +710,15 @@ describe("fixMissingThumbnails", () => {
         const client = await login(buildThumbFetch(fixMock));
         const lib = await openLib(client);
 
-        const results = await fixMissingThumbnails(lib, client, [101, 102]);
+        const errors: string[] = [];
+        const results = await fixMissingThumbnails(
+            lib,
+            client,
+            [101, 102],
+            (msg, level) => {
+                if (level === "error") errors.push(msg);
+            },
+        );
         lib.close();
 
         expect(results.length).toBe(2);
@@ -707,6 +726,10 @@ describe("fixMissingThumbnails", () => {
         const failure = results.find((r) => r.fileID === 102)!;
         expect(success.status).toBe("fixed");
         expect(failure.status).toBe("failed");
+        // The failed file, and only it, is reported as an error.
+        expect(errors).toEqual([
+            expect.stringMatching(/^\[.*\] FAILED file-102\.jpg: /),
+        ]);
     });
 
     it("skips a non-JPEG image and a video as unsupported, not failed (issue #17)", async () => {