Provide feedback to user when using the label command

What does this MR do and why?

When issuing the @gitlab-bot label ... command, the user didn't have any feedback whenever they provided a list of invalid labels, or one was not valid.

These changes allow the bot to provide a feedback message to the user, letting them know when an invalid issue was applied 🙂

Expected impact & dry-runs

The users will see a feedback message left from the bot when at least one issue is not valid.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review

Closes #798 (closed)