Subscriptions: Subscribing to comments silently fails for users with pending subscriptions block

### Impacted plugin

Jetpack

### What

Subscribing to comments should show a notification if subscribing failed due to a number of pending subscriptions.

### How

Users who subscribe to new posts via widget get notified if they have too many pending subscriptions blocking them from subscribing further. This was fixed in https://github.com/Automattic/jetpack/pull/14275.

If users with too many pending subscriptions attempt to subscribe to comment replies or new posts by using the "Notify me of new posts by email" checkbox, their subscription will silently fail. The comment will get posted, but they will never receive a subscription confirmation email.

Informing users of pending subscriptions problem as we do in a widget would be an improvement.