嵌套在 form 表单的组件销毁时，使用 w5c-dynamic-element 的元素会发生报错

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嵌套在 form 表单的组件销毁时，使用 w5c-dynamic-element 的元素会发生错误

angular.js:14791 TypeError: Cannot read property 'tagName' of undefined
    at getParentGroup (w5cValidator.js:23)
    at getParentGroup (w5cValidator.js:29)
    at getParentGroup (w5cValidator.js:29)
    at getParentGroup (w5cValidator.js:29)
    at getParentGroup (w5cValidator.js:29)
    at getParentGroup (w5cValidator.js:29)
    at validatorFn.defaultRemoveError (w5cValidator.js:59)
    at validatorFn.removeError (w5cValidator.js:152)
    at W5cFormValidateCtrl.removeError (w5cValidator.js:426)
    at w5cValidator.js:455

使用自定义错误可以规避这个错误

这个场景可能比较少人用到，且问题不大，不过还是发个 issue 让可能遇到同样问题的人看到