Manager: prevent logging an error when fetching unknown worker

Prevent logging an error in the persistence layer when an unknown worker
is requested.

This reduces the noise & confusion when the web interface is showing the
details of a worker, but the worker gets removed by someone else. Or when
the Manager doesn't know about a Worker and it's trying to connect.

See #104282.

internal/manager/persistence/workers.go

@@ -75,10 +75,14 @@ func (db *DB) FetchWorker(ctx context.Context, uuid string) (*Worker, error) {
 	w := Worker{}
 	tx := db.gormDB.WithContext(ctx).
 		Preload("Tags").
-		First(&w, "uuid = ?", uuid)
+		Find(&w, "uuid = ?", uuid).
+		Limit(1)
 	if tx.Error != nil {
 		return nil, workerError(tx.Error, "fetching worker")
 	}
+	if w.ID == 0 {
+		return nil, ErrWorkerNotFound
+	}
 	return &w, nil
 }