Retry failed notifications

[JamesHemery]

It would be nice to implement a retry (exponential backoff) system for failed batched notifications.

For non-batched notifications, it would be necessary to throw an exception or dispatch an event when the notification fails.

We talk about a failed notification when the payload is valid but Expo returns with a error status.

When the payload is invalid, Expo returns a body containing errors field and the Service throw an ExpoNotificationsException.

Also, we could consider dispatching an event when a Ticket or Receipt is in error status.