Improve default messages

When a user tries to subscribe to new comments for a topic that they have been already subscribed, an error message pops up: "Subscription Fault", which gives no clue what actually went wrong. It would make sense to return actual error description from the "addSubscription" method rather than just a subscriptionSuccess flag.

Next, if there were no errors, user sees a message "Confirm your subscription", which may be a little confusing (how to confirm?). I know it's possible to customize messages by your own, but it would be great to have the plugin be more user-friendy out of the box, so it could say by default something like "Check your email for a subscription confirmation link".