title: How to use <md-error> in Angular Material 2?
body: <p>I am trying to display an error message when a required input is not valid in Angular Material 2. There is an example in the <a href="https://github.com/angular/material2/blob/master/src/demo-app/input/input-demo.html" rel="nofollow noreferrer">Angular Material demo app</a> : </p>
  
  <pre><code>&lt;form novalidate&gt;
    &lt;h4&gt;Inside a form&lt;/h4&gt;
  
    &lt;md-input-container&gt;
      &lt;input mdInput name="example" placeholder="example"
        [(ngModel)]="errorMessageExample3" required&gt;
      &lt;md-error&gt;This field is required&lt;/md-error&gt;
    &lt;/md-input-container&gt;
  
    &lt;button color="primary" md-raised-button&gt;Submit&lt;/button&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/jljFc.png" rel="nofollow noreferrer">Expected result</a></p>
  
  <p>Unfortunately when I try to do the same, I get :</p>
  
  <pre><code>Unhandled Promise rejection: Template parse errors:
  'md-error' is not a known element:
  1. If 'md-error' is an Angular component, then verify that it is part 
     of this module.
  2. If 'md-error' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' 
     to the '@NgModule.schemas' of this component to suppress this message.
  </code></pre>
  
  <p>What am I supposed to do to use <code>&lt;md-error&gt;</code>?</p>