This introduces a table mapping OpenSSL's numeric verification codes to

our CertificateVerificationFailureCode enum. This helps to create the

right exceptions for an SSLError.
Python doesn't expose constants for any of these codes, so the numeric

values are used along with comments listing the OpenSSL error code's

typical constant.
CertificateVerificationFailureCode.for_ssl_verify_code() is used to

produce the right code based on the mapping. This will be used in

upcoming handling of SSLErrors.

Unit tests pass.