Allow mat-error in mat-form-field subviews

#### What is the expected behavior?
**NOTE**: it's more of an improvement (but a really critical one) than a bug.

`mat-error` should work even if it's not used as direct child of `mat-form-field`. It should work if it's used inside a component which itself is used in `mat-form-field`.

This is particularly necessary for **handling error messages in a reusable component**.
A real-world application may have several forms with a lot of validators on their fields.
Repeating a bunch of `mat-error`s and corresponding error messages for each usecase is just tedious.

### Suggested solution
I think the reason this currently doesn't work is because `@ContentChildren` decorator is used to get a hold of `mat-error` instances. However `@ContentChildren` [doesn't work for nested components](https://github.com/angular/angular/issues/5241) and aparently [this is by design](https://github.com/angular/angular/issues/5241#issuecomment-156841935).
Instead of using `@ContentChildren` , `MatError` can inject `MatFormField` and register itself into it.

#### What is the current behavior?
`mat-error` only works if it's directly used inside `mat-form-field`. If it's used inside a nested component, it doesn't work as expected.

#### What are the steps to reproduce?
https://stackblitz.com/angular/kllvoljyoem

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
@angular/cdk@7.3.0
@angular/material@7.3.0
Angular: 7.2.3

#### Is there anything else we should know?

Inizia con la riproduzione StackBlitz collegata e analizza come mat-form-field rileva le istanze di mat-error attraverso un componente riutilizzabile annidato. Conferma l'attuale limitazione ai figli diretti, quindi verifica che gli errori di convalida vengano visualizzati quando mat-error è annidato e che il comportamento esistente per i figli diretti rimanga invariato.

Scritto dal modello di indicizzazione a partire dal testo della issue.