Add typed Pi-hole v6 errors

[headdirt]

Summary

Add typed exceptions for v6 API error handling so callers can react to why a specific request has failed.

Changes

• New exception types (subclasses of HoleError):
  • HoleAuthenticationError - authentication required, missing, or invalid (HTTP 401)
  • HoleResponseError - unexpected or unparseable payload
• HoleError now carries optional status with the HTTP status code
• _fetch_data raises HoleAuthenticationError when a request is still 401 after the re-auth retry, and HoleResponseError when the body can't be parsed as JSON (existing session-expiry re-auth behavior is unchanged)
• authenticate raises HoleAuthenticationError on a 401, and HoleResponseError on an unparseable body
• Behavioral change: get_versions validates /info/versions and raises HoleResponseError instead of leaving versions empty
• Added tests

[fabaff]

Looks good to me.

Thanks