Made x509 parse error on CA certs more explicit.

This is helpful for diagnosing issues with old CA certs generated by GnuTLS which had invalid RSA modulus numbers (https://gitorious.org/gnutls/gnutls/commit/3d8da5765133c6ced37bf29b5a07f950b8c26cd7). We weren't actually aware of this issue with our old CA cert until provisioning logstash, so thanks! :smile:

In this specific case, error would now read:

    Failed to parse CA certificate /etc/ssl/cacert.pem - x509: RSA modulus is not a positive number