Improve validation errors for external CI/CD configuration

What does this MR do?

This merge request makes validation errors for external CI/CD config, included with include, more readable.

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#51369

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides