Improve download error failures

[ehennestad]

What changed

• improve error handling when versions.json and related metadata downloads fail
• raise a clear DownloadError instead of falling through to a later FileNotFoundError
• make SSL certificate verification failures report a clearer, more actionable message
• add versions.json to .gitignore so the downloaded file is not tracked accidentally

Why

When the validator could not download versions.json, it logged a network error and then continued into a confusing FileNotFoundError because the file was never created.

Impact

Validation failures caused by download problems should now be easier to understand and diagnose, especially for local SSL certificate issues.

Validation

• parsed openMINDS_validation/utils.py successfully with ast.parse
• did not run the full validator flow