Don't block PRs if no Slack user found
Sending Slack notifications for a PR author is a nice-to-have and can fail if:
- The author's
git config
isn't set appropriately - The author has set their commit email addresses to private
- Their Slack email address is an alias of their commit email address
- More
The change in this release won't block if no Slack user can be found for the PR's associated email address. This has the added benefit of making notifications more of an opt-in model as well.