[zerochan] handle "KeyError - 'items'" (#5826)

Zerochan sometimes sends an empty response when there are no more
accessible posts to be had.

gallery_dl/extractor/zerochan.py

@@ -183,7 +183,8 @@ def posts_api(self):
             data = self.request(url, params=params).json()
             try:
                 posts = data["items"]
-            except ValueError:
+            except Exception:
+                self.log.debug("Server response: %s", data)
                 return
 
             if metadata: