Handle non-200 status code in NVD response

[dcaravel]

Failures from NVD were not being handled nicely, leading to logs that resemble:

2025/12/15 18:09:00 could not validate NVD CVE CVE-2025-55315: unmarshalling NVD response result: invalid character 'e' looking for beginning of value

This particular error was likely rate limits being hit, with this change the error will instead resemble

2025/12/15 13:00:36 could not validate NVD CVE CVE-2025-21176: NVD API returned status 429 (Too Many Requests): "error code: 1015"