Log njsscan errors at the WARN level

What does this MR do?

Scan errors can occur when njsscan fails to analyse an individual file in a repository. This could be a timeout or some other error that should be plainly visible to the user, but shouldn't cause the entire scan to abort.

What are the relevant issue numbers?

• https://gitlab.com/gitlab-com/sec-sub-department/section-sec-request-for-help/-/issues/5

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer