* 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
