send Slack notification if e2e fails/on proceed or abort prompt

[vdice]

The e2e stage in the workflow-cli pipeline currently awaits (indefinitely) for input if the first e2e run failed, asking the user whether to proceed (with another e2e run) or abort. It would be helpful if a Slack message was broadcast, perhaps to #testing, ideally pinging the owner of the PR, on this event.

[Joshua-Anderson]

This is contingent on updating the slack plugin and the jobs-dsl plugin to get slack message support in jenkinsfiles.

[Joshua-Anderson]

#133 Uses email notifications to slack for a temporary stopgap measure instead of actual slack messages.

[vdice]

As @Joshua-Anderson mentioned, we've set up email notifications for now. We can re-open and/or create a new ticket to change over to notifications when necessary. Closing.