[exhentai] improve image limits check

- check if current image is the '509 Bandwidth Exceeded' notification
  (https://ehgt.org/g/509.gif or https://exhentai.org/img/509.gif)
- remove 'limits' option

docs/gallery-dl.conf

@@ -79,7 +79,6 @@
             "username": null,
             "password": null,
             "domain": "auto",
-            "limits": true,
             "metadata": false,
             "original": true,
             "sleep-request": 5.0