netops: return GIT_ECERTIFICATE when it fails the basic tests

When we first ask OpenSSL to verify the certfiicate itself (rather
than the HTTPS specifics), we should also return
GIT_ECERTIFICATE. Otherwise, the caller would consider this as a failed
operation rather than a failed validation and not call the user's own
validation.

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We really need to figure out the creation of a test CA and certs and http-hosting repos so we can automatically test this.