(Un)Subscribing to folder doesn't check return value from backend

[cwickert]

1. Make sure a folder cannot be subscribed (e.g. by disabling subscriptions for a namespace)
2. Go to Settings -> Folders
3. Subscribe to that folder
4. Roundcube displays an error message:
   Server Error: SUBSCRIBE: This namespace has no subscriptions (0.000 + 0.000 secs).
5. Nevertheless the checkbox is now checked and the subscription state is not returned correctly.
6. After reloading the page, the checkbox for the folder is no longer listed as subscribed.

Even though this is a corner case, I think roundcube should always check the actual state in step 4 rather than the js check/uncheck events.