fix: handle potential KeyError

When fetching a large number of personal documents, there is a chance of
receiving the following JSON structure leading to a KeyError:

```json
{"OwnershipData":{"success":false,"error":"GENERIC_ERROR"}}
```

In this case, ignore the error and continue execution.

kindle_download_helper/kindle.py

@@ -334,7 +334,11 @@ class Kindle:
             if not result.get("success", True):
                 logger.error("get all books error: %s", result.get("error"))
                 break
+            try:
                 items = result["OwnershipData"]["items"]
+            except KeyError:
+                logger.error("get all books error: %s", result.get("error"))
+                break
             for item in items:
                 if filetype == "PDOC":
                     item["title"] = html.unescape(item["title"])