Exit with 1 if fatal errors are found while validating the distribution manifest

[OneBlue]

Summary of the Pull Request

This change updates validate-modern.py to return 1 when a fatal error is found in the distribution manifest.

PR Checklist

• Closes: Link to issue #xxx
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed