postinstall script writes error response to domains.json instead of failing

My CI runners' IPs appear to be blocked by HubSpot's CDN, which returns [error code 1106](https://developers.cloudflare.com/support/troubleshooting/http-status-codes/cloudflare-1xxx-errors/error-1006/). The postinstall script writes this error to `domains.json` instead of failing, because it doesn't validate that the response is actually valid domain data. The script only catches network errors, not HTTP error status codes or malformed responses. The script should check that the HTTP status code is 200 before processing the response.