title: WebRTC how to set constraints to use audio when available
body: <p>Typically you would set constraints as follow</p>
  
  <pre><code>{ audio: true, video: true }
  </code></pre>
  
  <p>but I'd like to be more permissive then that. I'd like to only get the audio, if it is able to get it <em>i.e. if a microphone is available</em>, otherwise it will throw me a <code>NotFoundError</code> error.</p>
  
  <p>Any ideas if that's possible? </p>