Remove debug log that was part of some old development effort.
Ensure that when subscribing to a new feed, if some error happens no new feeds are saved in the database.

The ImportSubscriptionsJob does not delete feeds if an error happens when trying to import them. This is already handled by the UrlSubscriber class, which:

- ensures no new feeds are saved in the DB if an error happens while subscribing to a feed new for feedbunch, by destroying them in the error handler

- does not destroy an already existing feed if there's an error while trying to subscribe another user to it. The fact that there's been an error subscribing a user is not reason enough to destroy the feed and therefore remove it from all other users that were subscribed to it (it could be a transient error).