title: How to detect firestore connection failure
body: <p>I would like to know how it is possible with angularfire2 to know if the cloud firestore database is accessible and that I can get collection values?</p>
  
  <p>The goal here is if it fails, I will go get local data instead.</p>
  
  <p>I have something like</p>
  
  <pre><code>this.afStore.collection('messages').valueChanges().subscribe(
    (messages: any[]) =&gt; {
  
      // DO STUFF
    });
  </code></pre>
  
  <p>Thanks</p>