title: How to pass reactive form control from child to parent component in angular 2
body: <p>I Have nested reactive form, like below</p>
  
  <p>parent HTML</p>
  
  <pre><code>&lt;form&gt;
   &lt;child&gt;&lt;/child&gt;
   &lt;button mat-raised-button color="primary"&gt;Save&lt;/button&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>child HTML</p>
  
  <pre><code>&lt;div [formGroup]="headerSection"&gt;
   &lt;input type="text" formControlName="test"&gt;
   &lt;!span [hidden]="headerSection.get('test').valid"&gt;required&lt;/span&gt; 
  &lt;/div&gt;
  </code></pre>
  
  <p>I want to throw error message in child HTML, but when clicking the parent submit button...</p>