func NewHTTPError(code int, formatStr string, args ...interface{}) *HTTPError {
	return &HTTPError{
		code: code,
		err:  fmt.Sprintf(formatStr, args...),
	}
}
func NewHTTPError(code int, formatStr string, args ...interface{}) *HTTPError {
	return &HTTPError{
		code: code,
		err:  fmt.Sprintf(formatStr, args...),
	}
}
// NewHTTPError returns a new HTTPError where the HTTP error code is 'code' and
// the error message is based on 'formatStr' and 'args'