docs: Add registration error on firebase guide (#422)

pages/docs/v3/guides/push-notifications-firebase.md

@@ -360,6 +360,25 @@ import Firebase
 FirebaseApp.configure()
 ```
 
+Then you need to add the following two methods to correctly handle the push registration events:
+
+```swift
+func application(_ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {
+  Messaging.messaging().apnsToken = deviceToken
+  Messaging.messaging().token(completion: { (token, error) in
+    if let error = error {
+        NotificationCenter.default.post(name: .capacitorDidFailToRegisterForRemoteNotifications, object: error)
+    } else if let token = token {
+        NotificationCenter.default.post(name: .capacitorDidRegisterForRemoteNotifications, object: token)
+    }
+  })
+}
+
+func application(_ application: UIApplication, didFailToRegisterForRemoteNotificationsWithError error: Error) {
+  NotificationCenter.default.post(name: .capacitorDidFailToRegisterForRemoteNotifications, object: error)
+}
+```
+
 Your completed `AppDelegate.swift` file should look something like this:
 
 ```swift
@@ -378,21 +397,21 @@ class AppDelegate: UIResponder, UIApplicationDelegate {
     FirebaseApp.configure()
     return true
   }
-```
 
-If you would like to receive the firebase FCM token from iOS instead of the raw APNS token, you will need to also change your `AppDelegate.didRegisterForRemoteNotificationsWithDeviceToken` code to look like this:
+  func application(_ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {
+    Messaging.messaging().apnsToken = deviceToken
+    Messaging.messaging().token(completion: { (token, error) in
+      if let error = error {
+          NotificationCenter.default.post(name: .capacitorDidFailToRegisterForRemoteNotifications, object: error)
+      } else if let token = token {
+          NotificationCenter.default.post(name: .capacitorDidRegisterForRemoteNotifications, object: token)
+      }
+    })
+  }
 
-```swift
-    func application(_ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {
-        Messaging.messaging().apnsToken = deviceToken
-        Messaging.messaging().token(completion: { (token, error) in
-            if let error = error {
-                NotificationCenter.default.post(name: .capacitorDidFailToRegisterForRemoteNotifications, object: error)
-            } else if let token = token {
-                NotificationCenter.default.post(name: .capacitorDidRegisterForRemoteNotifications, object: token)
-            }
-          })
-    }
+  func application(_ application: UIApplication, didFailToRegisterForRemoteNotificationsWithError error: Error) {
+    NotificationCenter.default.post(name: .capacitorDidFailToRegisterForRemoteNotifications, object: error)
+  }
 ```
 
 ### Upload the APNS Certificate or Key to Firebase