Added email warning on books order error

* Added a "BookOrderErrorEvent" Event type and a BOOK_ORDER_ERROR Event code
* Renamed "BookOrderOperationEvent" & "BookShippingOperationEvent" to "BookOrderEvent" & "BookShippingEvent"
* Added a Twig template, i18n data & email data params for a new "book order error" email
* The Twig templates of emails sent to the website admin are now stored in a "admin/" subdirectory
* Stripe errors are now logged