func (e *Error) Error() string {
	if e.Msg != "" {
		return e.ErrType + ": expr_path=" + e.FailField + ", cause=" + e.Msg
	}
	return e.ErrType + ": expr_path=" + e.FailField + ", cause=invalid"
}
Error implements error interface.
package binding
type Error struct {
	ErrType, FailField, Msg string
}