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-errors 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 and aparently this is by design.
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?

Start with the linked StackBlitz reproduction and inspect how mat-form-field discovers mat-error instances across a nested reusable component. Confirm the current direct-child limitation, then verify that validation errors render when mat-error is nested and that existing direct-child behavior remains intact.