There seems to be several issues when an error is being thrown in the modal.

1. When an error occurs while setting up the modal's body the same error notification is displayed twice. From a discussion with dobedobedoh:
   "One of the issues is that setBody() has its own promise chaining. It uses .then().then().fail(Notification.exception).always().fail(Notification.exception)}}The problem with jQuery's {{.fail() is that it returns a rejected Promise which means you get two notifications with the same error"
2. The modal backdrop is not being cleared once the displayed error notifications and the existing modals are closed. This blocks users to take any actions on the page and are forced to refresh the page.