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 #18

src/kobuddy/__init__.py

@@ -77,6 +77,7 @@ def bid(self) -> str:
 
 
 if typing.TYPE_CHECKING:
+    # todo: since 3.8 can use from typing import Protocol everywhere
     from typing_extensions import Protocol
 else:
     Protocol = object
@@ -324,7 +325,19 @@ def all(self) -> List[Book]:
                 bset.update(l)
         return list(sorted(bset, key=lambda b: b.title))
 
-    def add(self, book: Book):
+    def make_orphan_book(self, *, volumeid: str) -> Book:
+        # sometimes volumeid might be pretty cryptic, like a random uuid (e.g. for native kobo books)
+        # but there isn't much we can do about it -- there isn't any info in Bookmark table
+        orphan = Book(
+            title=f'<DELETED BOOK {volumeid}>',
+            author='<kobuddy>',
+            content_id=volumeid,
+            isbn='fake_isbn_{volumeid}',
+        )
+        self.add(orphan)
+        return orphan
+
+    def add(self, book: Book) -> None:
         Books._reg(self.cid2books, book.content_id, book)
         Books._reg(self.isbn2books, book.isbn, book)
         Books._reg(self.title2books, book.title, book)
@@ -842,6 +855,7 @@ def _iter_highlights(**kwargs) -> Iterator[Highlight]:
     logger = get_logger()
     books = _get_books()
 
+    # todo more_itertools?
     yielded: Set[Highlight] = set()
     for bfile in DATABASES:
         for h in _load_highlights(bfile, books=books):
@@ -856,12 +870,19 @@ def _load_highlights(bfile: Path, books: Books):
     db = dataset_connect_ro(bfile)
     for bm in db.query('SELECT * FROM Bookmark'):
         volumeid = bm['VolumeID']
-        book = books.by_content_id(volumeid)
-        assert book is not None, bm
-        # TODO make defensive?
-        # TODO could be example of useful defensiveness in a provider
+        mbook = books.by_content_id(volumeid)
+        if mbook is None:
+            # sometimes Kobo seems to recycle old books without recycling the corresponding bookmarks
+            # so we need to be a bit defensive here
+            # see https://github.com/karlicoss/kobuddy/issues/18
+            book = books.make_orphan_book(volumeid=volumeid)
+        else:
+            book = mbook
+        # todo maybe in the future it could be a matter of error policy, i.e. throw vs yield exception vs use orphan object vs ignore
+        # could be example of useful defensiveness in a provider
         yield Highlight(bm, book=book)
 
+
 def get_highlights(**kwargs) -> List[Highlight]:
     return list(sorted(_iter_highlights(**kwargs), key=lambda h: h.created))