Add problem resolution text to redirects Rake task

What does this MR do and why?

While working on: technical-writing#900 (closed), I encountered a fatal error with some malformed metadata.

It wasn't obvious from the output of the Rake tasks that it was a fatal error, so I raised an MR and only realized later that my results were partial.

This MR adds some problem resolution text to the Rake task. Example output:

ERROR: No redirect_to found in ../gitlab/doc/user/project/import/phabricator.md!

To resolve the problem:
1. Add redirect_to to metadata of ../gitlab/doc/user/project/import/phabricator.md.
2. Raise merge request with metadata change and have the change merged.
3. Run Rake task again.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. DRY_RUN=true bundle exec rake docs:clean_redirects.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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