Added some safeguards for comment syncing

I can't reproduce #2391, but I've added a couple things that might help
avoid crashing:

- Use the blog's object ID on the comment sync callback. I've tried
  deleting a blog while it was syncing comments, and got a completely
  different crash, but this is the right way anyway.
- Use a fetch request instead of `blog.comments`.
- Add some logging if the comment can't be found