Add basic KV error map testing with mock

As an off-by-default feature test since we don't want to require the mock for all testing, add a feature test that at a minimum pulls the kverror map from the mock and sets up the error codes described.

Intent here is that Couchbase developers will turn this on in their testing.

A follow-on issue will require the addition of feature tests against the mock that exercise various behaviors required in the KVError Map.