This updates most of our certificate-related error handlers to catch

CertificateVerificationError and to return information on the

certificate.
Most of this is in the API. We're re-using the UNVERIFIED_HOST_CERT

for the response. Previously, in most cases, SSL errors were treated as

generic SCMErrors, so this will be a change for callers, but is not

considered an API breakage.
There are other places in the codebase that still need to handle this

error, and will be updated as more of the SSL cert code is completed.

All unit tests pass.