This introduces CertificateVerificationError, which represents an

unverified certificate, and VerificationFailureCode, which is an enum

of handled error codes. These will be used product-wide to represent

verification failures, replacing the now-deprecated

reviewboard.scmtools.errors.UnverifiedCertificateError.
The new error class provides default error messages for different error

codes, and allows for some custom formatting containing certificate

information.

Unit tests passed.
Made use of this in in-progress code.