Ensure rspec helpers call curl with --fail

What does this MR do and why?

If a curl request fails with a 404, we should default to the fallback routine so tests like detect-test will gracefully recover.

Relates to Failed `detect-test` job due to missing fronten... (#473318 - closed)

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Before	After

How to set up and validate locally

1. Check detect-tests job while https://gitlab-org.gitlab.io/gitlab/crystalball/frontend_fixtures_mapping.json is returning a 404.