bug(mat-form-field): mat-error not displayed when form field is in an error state.

#### Reproduction

The Stackllitz app (https://stackblitz.com/edit/yz1dhz-m5re6l) renders two form controls in a form group:

1. A [mat-select](https://material.angular.io/components/select/overview)
2. A [custom form field control](https://material.angular.io/guide/creating-a-custom-form-field-control) (wraps Codemirror)

Both form controls are immediately invalid because they are initailized with null and have a required validator. The error message for each form control is displayed in ``.

Steps to reproduce:
1. Select "Foo" from the `` form control to set the corresponding validator on the second form control (the custom form field).
3. The error message for the second form control is correctly displayed `{ "required": true, "message": "Must be Foo" }`.
4. Enter the value "Foo" into the second form control to move the form control to a valid state.
5. The `` is shown. All good so far :)
6. Select "Bar" from the first form control to set a different validator.
7. The second form control is in an error state and the form field is displayed with a solid red color underline.
8. `` is NOT displayed and the `` is.


#### Expected Behavior

`` is hidden and `` is displays `{ "message": "Must be Bar" }`

#### Actual Behavior

`` is hidden and `` is displays `I am a hint.`

#### Environment

- Angular: 9.1.6
- CDK/Material: 9.2.3
- Browser(s): Chrome Version 83.0.4103.116 (Official Build) (64-bit)
- Operating System (e.g. Windows, macOS, Ubuntu): ArchLinux

リンクされた StackBlitz の再現から始め、mat-select と Codemirror をラップするカスタムフォームフィールドコントロールを比較します。validator の変更後に mat-form-field が mat-error と mat-hint のどちらを選択するかを追跡します。"Must be Bar" というエラーメッセージが表示され、hint が非表示になれば完了です。

索引モデルが issue の本文から書いたものです。