Fix minor issues with email config (#6962)

 * Give `notif_template_html`, `notif_template_text` default values (fixes #6960)
 * Don't complain if `smtp_host` and `smtp_port` are unset, since they have sensible defaults (fixes #6961)
 * Set the example for `enable_notifs` to `True`, for consistency and because it's more useful
 * Raise errors as ConfigError rather than RuntimeError for nicer formatting