Unwrap error in Amazon validator

In amazon/validator.go, an error was being wrapped by fmt.Errorf("%v",
err). That discards any type information about the returned error;
returning err lets the callers inspect the exact error returned by the
HTTP client.

The test had to change to do similar inspection.

amazon/validator.go

@@ -95,7 +95,7 @@ func (c *Client) Verify(ctx context.Context, userID string, receiptID string) (I
 
 	resp, err := c.httpCli.Do(req)
 	if err != nil {
-		return result, fmt.Errorf("%v", err)
+		return result, err
 	}
 	defer resp.Body.Close()