[Direct Transfer] Do not cache folders in error in the remote folders tree

In the Direct Transfer window, the list of remote folders can be gray. This happens when:

• case 1: the folder is read-only for the user
• case 2: there was an error when fetching the folder metadatas

As folder "items" (item = Qt element representing the folder in the view) are cached, when the user will try another Direct Transfer (without restarting the application), then the folders list will not be updated and still gray.

In the second case, the item should not be cached at all.