Post `review-deploy` failures to a Slack channel

The following discussion from gitlab-ce!32223 should be addressed:

• @kwiebers started a discussion: (+8 comments)

  @rymai - Prior to the --atomic change, did you do any regular resource cleanup that you were doing manually?

  If not, I say we merge this in as is and we can always adjust based on what we find. I can own some analysis with the whole GitLab chart in a separate gitlab-ce~2936423 issue. I just don't want to hold this up for a rabbit hole I was going down 😆

  WDYT?