XWIKI-11521: Send 3 new events for the email feature, mail ready to be s...

...ent, mail sent, mail error
Should this also be [`Cancelable`](https://github.com/xwiki/xwiki-commons/blob/master/xwiki-commons-core/xwiki-commons-observation/xwiki-commons-observation-api/src/main/java/org/xwiki/observation/event/CancelableEvent.java), in which case if the event is stopped the email is not sent?