Do not raise exception when artifact is not found

What does this MR do and why?

Closes Missing notification to master-broken for maste... (#1501 - closed)

Expected impact & dry-runs

Tested with gitlab-org/quality/engineering-productivity/master-broken-incidents-playground#28 (closed)

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