Handle invalid AMEX CV2 number gracefully

[codeinthehole]

Sample response:

<?xml version="1.0" ?>
<Response>
    <CardTxn>
        <card_scheme>American Express</card_scheme>
    </CardTxn>
    <datacash_reference>390010290042743</datacash_reference>
    <information>Expected 4 digit CV2 for American Express</information>
    <merchantreference>103483_PRE_2_6910</merchantreference>
    <mode>LIVE</mode>
    <reason>Invalid CV2</reason>
    <status>5</status>
    <time>1386788868</time>
</Response>