If there occures an error when sending a customer mail, the seller ist not informed,
because the error results in an error page being shown and the process stops.
The order will still be saved but neigther the customer nor the seller get an information.

I propose to put a catch around sending the customer mail and stop the event proagation.
It might be possible to forward the information to the template so that the customer and seller
might be informed of the failed mailing process.

Locations:
Classes/Controller/Order/OrderController.php
Classes/EventListener/Order/Finish/Email.php