Renamed app/exceptions to app/errors.

This is more correct, because the classes there actually inherit from StandardError.
Implemented method to retrieve feeds in a folder from a given user.
WIP: unit tests for FeedsController#index, when a folder_id param is passed.
Fixed bug that made FolderManager.folder_feeds return all subscribed feeds with unread entries when a false was passed in the include_read argument. The correct behavior is to return only feeds with unread entries **in the passed folder**.
Fixed unit tests for FeedsController#index
When unsubscribing, if there were no more feeds in the folder besides the unsubscribed feed, the folder is immediately removed from the scope.

This affects both folders in the sidebar and the folders dropdown.

Since unsubscribing is not immediate but rather happens after a certain delay in a background job, the angularjs client requests from the server a list of feeds in the folder, and if the only feed returned is the unsubscribed one, the folder is removed from the client (the actual removal from the db will happen when the job is performed). This is a case of "fake it till you make it" which should feel responsive to users in most cases.
When unsubscribing, request all feeds in the folder when trying to determine if the folder should be removed, instead of requesting only feeds with unread entries in the folder.

If there are other feeds in the folder besides the just unsubscribed one, the folder should be present in the scope regardless of whether those feeds have unread entries or not.

This is because all folders should appear in the dropdown, so that the user has the option of moving any feed into any of his existing folders. The sidebar is another story: only folders and feeds with unread entries should appear there (unless the "show read" button has been clicked), but the view takes care of filtering and hiding folders in the sidebar, instead of removing them from the scope.