Handle success with warnings

@maxxer Many thanks for this! :thumbsup: 

Having read up in the PayPal docs more about this, I've added this functionality to `setExpressCheckout`, `getExpressCheckoutDetails` and `doExpressCheckout`.

Do you happen to have one of the original responses that failed? Or know if either `L_ERRORCODE0` or `L_LONGMESSAGE0` were set? 

(or tell me how to reproduce and I'll check)