Don't raise when pipeline failure config isn't found

What does this MR do and why?

We needlessly raise when no config is found, but it's ok to not find any config as most pipeline shouldn't result in incident/Slack notification.

We can see such errors at https://sentry.gitlab.net/gitlab/triage-ops/issues/4120588/?environment=production.

Expected impact & dry-runs

No error raised when no pipeline failure config is found.

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