Notification Rule: If sending to slack, and the url is incorrect, the UI should report that the notification rule has failed

feedback from a user: When i had entered the wrong slack URL into the NotificationEndpoint, there was no ability for me to test that url. Then when I looked at the history of the notificationRules being run using that endpoint, there was no indication that the call to slack had failed due to a bad url. All i saw were green checkmarks.

The UI should show an error if the NotificationRule failed to notify due to a bad status code being returned, and give the user the ability to see that.

Start with the NotificationEndpoint and notificationRules history paths described in the report, then trace how a Slack request's status code reaches the UI. Reproduce a notification using an incorrect URL and verify that the UI reports the failed rule and makes the failure visible in its history.

Written by the indexing model from the issue text.