Improve backup.marker error message for rsync_tmbackup importer

[hiijoshi]

Summary

Improved the error message shown when backup.marker is missing in the rsync_tmbackup importer.

Changes

• Updated the FileNotFoundError message to clarify that backup.marker must exist inside the provided root directory
• Included the root path in the error message for easier debugging

Reason

This makes the error easier to understand and helps users quickly identify which directory is missing the required marker file.

[ThomasWaldmann]

good change. but maybe do PRs a bit differently in general as there is a huge overlap in the way you do it now:

if you do 2 independent changes (like working on the docs vs. improving the code), do 2 PRs, one only with the docs changes, one only with the code changes.

if a change depends on a previous PR, you can include the commits of the previous PR in the next PR (and maybe add a PR comment like "this PR depends on (previous PR)".

[hiijoshi]

Thanks for the feedback!

I’ve cleaned up this PR to include only the error message improvement for the rsync_tmbackup importer.

The other changes (tests and docs) are kept in their respective separate PRs.

Please let me know if this looks better.