Update with-firebase-auth example (#5742)

- The with-firebase-auth example won't run unless a database with proper rules is created first. 
- Add a small error callback to addDbListener to help with debugging if someone's database connection doesn't work.

examples/with-firebase-authentication/README.md

@@ -29,6 +29,7 @@ Set up firebase:
 - Get your authentication credentials  from the Firebase console under *authentication>users>web setup*. It will include keys like `apiKey`, `authDomain` and `databaseUrl` and it goes into your project in `credentials/client.js`.
 - Copy the `databaseUrl` key you got in the last step into `server.js` in the corresponding line.
 - Back at the Firebase web console, go to *authentication>signup method* and select *Google*.
+- Create a database in the "Database" tab and select the realtime database. Then go to "rules" and set up your write, read rules. Examples can be found here: https://firebase.google.com/docs/database/security/quickstart#sample-rules
 
 Install it and run:
 

examples/with-firebase-authentication/pages/index.js

@@ -59,6 +59,8 @@ export default class Index extends Component {
     firebase.database().ref('messages').on('value', snap => {
       const messages = snap.val()
       if (messages) this.setState({ messages })
+    }, (error) => {
+      console.error(error)
     })
   }