fix: Prevent nil deref in downloadFile when checking HTTPResponse on error

[vtjl10]

Summary: Adds a nil check for resp.HTTPResponse before accessing StatusCode in cmd/nitro/init.go to prevent a panic on transport errors.

• Behavior: Still returns notFoundError on 404, but only when an HTTP response is present; otherwise logs the attempt and continues retry logic.
• Rationale: Improves robustness of the init download path; avoids process crashes under transient network failures.

[Tristan-Wilson]

I believe HTTPResponse can be empty here in case of connection issues, so this will prevent a nil deref in the downloader.

[vtjl10]

@Tristan-Wilson how I can fix this?