CI: improve error reporting mechanism

[yannbf]

Issue:

What I did

• Improved the discord report message based on workflow, job, branch:
  • For the alpha branch, it will provide the Storybook version
  • For the chromatic-sandboxes job, it will ask the maintainer to check the visual diff, as it might not be a bug
  • For merged workflow, it will only show a single PR in the relevant PRs section
  • For the rest, which will be the daily workflow, it will show PRs from the past 24 hours
• Added reporting for all steps

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

If your answer is yes to any of these, please make sure to include it in your PR.