Add html variant for the notification emails
Add a TemplateRenderException

This allows the template rendering to catch known rendering issues
and handle them accordingly. Such as having no record of the
notification recipient. All other exceptions now pass through.
Raise the error if a notification fails to send
Add tests to validate the email notification content
Ensure wording is consistent in notification email
Do not catch any exceptions in the notifier
Fix example notification email text

Remove unused template variables.