tardev-snapshotter: fix regression to allow no signatures

[jiria]

Merge Checklist

• Followed patch format from upstream recommendation: https://github.com/kata-containers/community/blob/main/CONTRIBUTING.md#patch-format
• Included a single commit in a given PR - at least unless there are related commits and each makes sense as a change on its own.
• Merged using "create a merge commit" rather than "squash and merge" (or similar)
• genPolicy only: Builds on Windows
• genPolicy only: Updated sample YAMLs' policy annotations, if applicable

Summary

Fix recent regression that started to require signatures. In the preview mode, we want to start the containers even when no signatures can be found.

Refactors the code gently to simplify error reporting.

Associated issues

Links to CVEs

Test Methodology