title: Catch in Angularfire2 v5 error
body: <p>I am converting the use of Firebase Storage to use the Angularfire2 library (currently v5.0.0-rc.5-next) which means I am now using observables rather than promises.</p>
  
  <p>How can I catch error such as <code>storage/object-not-found</code> and react accordingly?</p>
  
  <p>This is currently my code but I cannot add a <code>catch</code> to it as some examples I found.</p>
  
  <pre><code>const avatarRef =  this.afStorage.ref('${userId}/avatar/${this.avatarThumbnail}${user.avatar}');
  
  avatarRef.getDownloadURL()
      .take(1)
      .subscribe((avatarUrl) =&gt; {
          resolve(avatarUrl);
      });
  </code></pre>