Consider messaging requirements for notification when a QA validation step is a blocker for deploying RC's

There does not appear to be a standard way of notifying anyone when an engineer finds a blocking bug that would be bad enough to prevent an RC from being deployed. Communication inside of issues may not always reflect the severity of a newly found bug. Utilize this issue to figure out how best to approach the necessary communication during the QA testing phase.

Reference (internal slack messaging): https://gitlab.slack.com/archives/C0XM5UU6B/p1557174400163100

Hi, there. We spotted a blocking issue during RC1 QA on staging but looks like it went on production anyway. How were we supposed to prevent that on top of following the guidelines exposed in the issue description [1].

[1]: https://gitlab.com/gitlab-org/release/tasks/issues/778#process

/cc @gitlab-org/delivery