Fail fast when both cert auth and open bucket password are used

Based on the RFC and in practice its ambiguous if both a bucket password and client cert auth are used - as in:

        // Use the CertificateAuthenticator to authenticate

        cluster.Authenticate(gocb.CertificateAuthenticator{})

        // Open the bucket

        bucket, err := cluster.OpenBucket("pixels", "blarb")

        if err != nil {

                fmt.Println("ERROR OPENING BUCKET:", err)

        }

Most certainly this is not intended and should be reported as an error - in java and .net this is the Mixed authentication exception, which also happens if you specify RBAC creds and a bucket password.