This is called when we receive a response code from Market for a RequestPurchase request that we made. This is NOT used for any purchase state changes. All purchase state changes are received in #onPurchaseStateChange(PurchaseState, String, int, long). This is used for reporting various errors, or if the user backed out and didn't purchase the item. The possible response codes are: RESULT_OK means that the order was sent successfully to the server. The onPurchaseStateChange() will be invoked later (with a purchase state of PURCHASED or CANCELED) when the order is charged or canceled. This response code can also happen if an order for a Market-managed item was already sent to the server. RESULT_USER_CANCELED means that the user didn't buy the item. RESULT_SERVICE_UNAVAILABLE means that we couldn't connect to the Android Market server (for example if the data connection is down). RESULT_BILLING_UNAVAILABLE means that in-app billing is not supported yet. RESULT_ITEM_UNAVAILABLE means that the item this app offered for sale does not exist (or is not published) in the server-side catalog. RESULT_ERROR is used for any other errors (such as a server error).
