fix for missing books during annotations processing

make annotations defensive when the source book doesn't exist

seems that it legit can happen, now kobuddy will create a special 'orphan' book object in this case
e.g.

```
17 Aug 2018 08:58 <DELETED BOOK file:///mnt/onboard/Feynman, Richard/QED_ The Strange Theory of Light and Matter_ The Strange Theory of     Light and Matter - Richard Feynman.epub> by <kobuddy>
     . When we add up only the arrows that point more or less to the right, we get a series of dips and a substantial final arrow—accordi    ng to the theory, we should now have a strong reflection! And indeed, we do—the theory is correct! Such a mirror is called a diffraction     grating, and it works like a charm.
```

sometimes it's more cryptic though, e.g.: but there isn't much we can do about it, there isn't any other identifying information in Bookmark table

```
17 Aug 2022 14:06 <DELETED BOOK dd28521e-572e-4dbf-9f43-d1d779a18caf> by <kobuddy>
      By learning and ultimately understanding, we can find ways to create space for our authentic selves in the digital age
         How?
```

workaround for https://github.com/karlicoss/kobuddy/issues/18