Rephrase policy bot message errors

What does this MR do and why?

This MR rephrases policy bot message errors for artifacts_missing errors.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a project without .gitlab-ci.yml configuration
2. Create a merge request approval policy with any scan_finding and license_scanning rules
3. Create a merge request
4. Verify the updated error messages

Related to #462978 (closed)