title: why error has not set in formarray custom validation valuechanges
body: <p>I have a below demo app and trying to set the errors in custom validators why it is not setting.</p>
  
  <p>Step:
  click on add users and type the value .
  On value changes , i have call the customer validator and set the error or return the error but it is not set.</p>
  
  <p>demo:
  <a href="https://stackblitz.com/edit/github-2mdrme-2qgzib?file=src%2Fapp%2Fapp.component.ts" rel="nofollow noreferrer">demos</a></p>
  
  <pre><code>import { Component, VERSION } from '@angular/core';
  import { FormArray, FormBuilder, FormGroup ,AbstractControl,Validators} from '@angular/forms';
  @Component({
    selector: 'my-app',
    templateUrl: './app.component.html',
    styleUrls: ['./app.component.css']
  })
  export class AppComponent {
     myForm: FormGroup;
  
    constructor(private fb: FormBuilder) { }
  
    ngOnInit() {
      this.myForm = this.fb.group({
        email: '',
        users: this.fb.array([])
      })
    }
  
    get userForms() {
      return this.myForm.get('users') as FormArray
    }
  
    getColors(index) {
      return this.userForms.get([index, 'colors']) as FormArray;
    }
  
    addUser() {
      const userGroup = this.fb.group({
        user: [],
        colors: this.fb.array([])
      })
  
      userGroup.controls['user'].setValidators(this.validateMobileNo);
  
      this.userForms.push(userGroup);
    }
  
    deleteUser(i) {
      this.userForms.removeAt(i);
    }
  
    addColor(index: number) {
      const colorGroup = this.fb.group({
        color: []
      })
  
      this.getColors(index).push(colorGroup);
    }
  
    deleteColor(userIndex: number, colorIndex: number) {
      this.getColors(userIndex).removeAt(colorIndex)
    }
  
  
  
  
    validateMobileNo(controller: AbstractControl): { [key: string]: any } {        console.log(controller.value);
  
  
  
      console.log("Returning null");
      controller.setErrors({error: "ghhggh"});
      return {error: "ghhggh"};
    }
  }
  </code></pre>