Return an API error when the cert can't be fetched
Return an API error when the cert can't be fetched

It looks like https://developers.cloudflare.com/ssl/e2b9968022bf23b071d95229b5678452/origin_ca_rsa_root.pem and https://developers.cloudflare.com/ssl/0d2cd0f374da0fb6dbf53128b60bbbf7/origin_ca_ecc_root.pem no longer exists (aka they return a 404).

This is causing users of the OriginCARootCertificate to use the HTML as a cert which is causing some issue and hand to debug error messages.
For this reason a check is added to ensure that the returned response is 200.

cloudflare/cloudflare-go,cloudflare/cloudflare-go