on bad config, either notify on slack and ignore new config or exit altogether

[yasunariw]

For Slack to send messages via webhooks, each channel mentioned in the repo config file must have a corresponding webhook defined in the secrets file.

Suppose a bot has been configured to use webhooks. The repo config is updated to introduce some new channels, but the user forgot to update the secrets file with the new webhook urls.

Every time a new config is loaded from a repo, the bot should validate it against its secrets file and send a Slack alert or exit.