Webhook KATA validation issue

[Hildy]

On a customer's webhook, it allowed them to save the KATA as:

automation/updateJiraIssueRecord
  uri: cerb:automation:example.webhook.respond
  disabled@bool: no

(note the missing : on the first line).

This led to a rather cryptic 500 internal error because ControllerWebhooks::respond() doesn't confirm that there are any valid handlers before passing it off to DevblockUiEventHandler::handleOnce().

Validation should catch that initial missing colon.