Return errors from JSON encode/decode

I found two JSON errors being shadowed; the one in amazon/validator.go
is being hidden by errors.New(responseError.Message) -- which should be
an empty string if there's a JSON error. So, it wouldn't report success,
but this gives the caller better information on what failed.

The second is in appstore/validator.go, which was ignoring encode errors
before POSTing a verify request.

appstore/validator.go

@@ -98,7 +98,9 @@ func NewWithClient(client *http.Client) *Client {
 // Verify sends receipts and gets validation result
 func (c *Client) Verify(ctx context.Context, reqBody IAPRequest, result interface{}) error {
 	b := new(bytes.Buffer)
-	json.NewEncoder(b).Encode(reqBody)
+	if err := json.NewEncoder(b).Encode(reqBody); err != nil {
+		return err
+	}
 
 	req, err := http.NewRequest("POST", c.ProductionURL, b)
 	if err != nil {