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/cli/log.test.ts

@@ -173,13 +173,13 @@ const FILES = "100\timage\tbeach.jpg\n";
 // line for the one album whose files it fetched. Debug adds the start and end
 // of the refresh, and trace each request with the status of its answer.
 const INFO = "Fetched the files of 1 of 1 collections\n";
-const DEBUG = "refresh started\n" + INFO + "refresh done\n";
+const DEBUG = "Refresh started\n" + INFO + "Refresh done\n";
 const TRACE =
-    "refresh started\n" +
+    "Refresh started\n" +
     "GET https://api.ente.io/collections/v2?sinceTime=0: HTTP 200\n" +
     "GET https://api.ente.io/collections/v2/diff?collectionID=1&sinceTime=0: HTTP 200\n" +
     INFO +
-    "refresh done\n";
+    "Refresh done\n";
 
 describe("the log level", () => {
     it("is info by default", async () => {
@@ -326,6 +326,7 @@ describe("what the library reports", () => {
         ]);
     });
 
+    // Each operation is named in plain words, as the README names it.
     it("logs a refresh's progress at info, and its start and end at debug", () => {
         const { log, lines } = collect();
         const onProgress = logRefresh(log);
@@ -342,13 +343,15 @@ describe("what the library reports", () => {
             status: "failed",
             error: "HTTP 503",
         });
+        onProgress({ operation: "fetchMLData", status: "started" });
         onProgress({ operation: "fetchMLData", status: "done", fetched: 3 });
 
         expect(lines).toEqual([
-            "debug: refresh started",
+            "debug: Refresh started",
             "info: Fetched the files of 1 of 2 collections",
-            "debug: refresh failed: HTTP 503",
-            "debug: fetchMLData done",
+            "debug: Refresh failed: HTTP 503",
+            "debug: ML data fetch started",
+            "debug: ML data fetch done",
         ]);
     });
 });