Fix 500 error when artifact file is missing

What does this MR do and why?

• Contributes to #386184 (closed)
• Sentry error: https://sentry.gitlab.net/gitlab/gitlabcom/issues/4034705

Problem

When CI::Build does not have an archived artifact, then the server returns a 500 error.

Solution

Validate the artifact file presence and return 404 if it's missing.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.