Expose more information through log.txt when log is missing.

app/lib/frontend/handlers/package.dart

@@ -230,7 +230,7 @@ Future<shelf.Response> packageScoreLogTxtHandler(
   }
   final log = await taskBackend.taskLog(package, version);
   return shelf.Response(
-    log == null ? 404 : 200,
+    log == null || log.startsWith('no log -') ? 404 : 200,
     body: log ?? 'no log',
     headers: {
       'content-type': 'text/plain;charset=UTF-8',

app/lib/task/backend.dart

@@ -960,7 +960,7 @@ class TaskBackend {
 
   /// Get log from task run of [package] and [version].
   ///
-  /// Returns `null`, if not available.
+  /// If log is unavailable, it returns some information about the internal state.
   ///
   /// If log is unavailable it's usually because:
   ///  * package is not tracked for analysis,
@@ -973,13 +973,21 @@ class TaskBackend {
   Future<String?> taskLog(String package, String version) async {
     final data = await gzippedTaskResult(package, version, 'log.txt');
     if (data == null) {
-      return null;
+      final status = await packageStatus(package);
+      if (status.runtimeVersion == null) {
+        return 'no log - `PackageState` entity missing';
+      }
+      final v = status.versions[version];
+      if (v == null) {
+        return 'no log - version is not tracked';
+      }
+      return 'no log - current version status: ${v.status}';
     }
     try {
       return utf8.decode(gzip.decode(data), allowMalformed: true);
     } on FormatException catch (e, st) {
       _log.shout('Task log for $package/$version is malformed', e, st);
-      return null;
+      return 'no log - `log.txt` contains malformed characters';
     }
   }
 

app/test/task/task_test.dart

@@ -77,7 +77,7 @@ void main() {
 
     // Check that the log is missing.
     final log1 = await taskBackend.taskLog('oxygen', '1.2.0');
-    expect(log1, isNull);
+    expect(log1, 'no log - version is not tracked');
 
     // Check that the dartdoc is missing
     final dartdoc1 = await taskBackend.dartdocFile(