utils: add trySlackSend() function, use it across the tree

[dustymabe]

This will make sending a slack message conditional on if the credential exists.

[dustymabe]

broken out from #656 for us to consider later

[jlebon]

If the goal is to make it easier to use Slack from non-official pipelines, I'd say we should make the room a config knob instead, which we can do by removing room from slack.yaml, add it to config.yaml, and pass it when calling slackSend.

And possibly refuse to post to Slack if the official channel name is used in non-official.

[dustymabe]

Something like this was merged in 33a1a58