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.

amazon/validator.go

@@ -102,6 +102,9 @@ func (c *Client) Verify(ctx context.Context, userID string, receiptID string) (I
 	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
 		responseError := IAPResponseError{}
 		err = json.NewDecoder(resp.Body).Decode(&responseError)
+		if err != nil {
+			return result, err
+		}
 		return result, errors.New(responseError.Message)
 	}