Make Keberos result codes available from errors

[lorenz]

This exposes inner errors in Krberror errors as a non-breaking change by making Krberror implement the Unwrap interface.

Example:

err := client.Login()
var krbErr messages.KRBError
if errors.As(err, &krbErr) {
    switch krbErr.ErrorCode {
        case errorcode.KDC_ERR_PREAUTH_FAILED:
            fmt.Println("This is a PREAUTH_FAILED error")
        }
}

Fixes #437

[jcmturner]

Without updating the Add method we don't capture the inner errors in all cases. krberror was written before Go implemented error wrapping. I'm thinking that the Add interface will have to change to implement the way Go wraps errors. This would therefore need a major version iteration.