loader: Raise NotFound when CVS repo no longer fetchable with rsync

When attempting to fetch CVS repository data using rsync protocol but that process exits with return code 23, it means the repository is no longer available so NotFound must be raised.

Fixes #4572 (closed) and SWH-LOADER-CVS-3K

It seems some CVS repositories listed by the SourceForge lister got removed since then so these errors.

Jenkins job DLDCVS/gitlab-builds #11 succeeded .
See Console Output and Coverage Report for more details.

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merged