Throw specific error on FCM error over generic

When there is an FCM error, it always throws an ExecutionException type
instead of the root cause error due to the Task.await used. To fix this
we need to catch this and use task.getException() to get the root cause.

https://developers.google.com/android/guides/tasks

Author: jkasten2

OneSignalSDK/onesignal/src/main/java/com/onesignal/PushRegistratorFCM.java

@@ -103,7 +103,7 @@ String getProviderName() {
 
    @WorkerThread
    @Override
-   String getToken(String senderId) throws ExecutionException, InterruptedException, IOException {
+   String getToken(String senderId) throws Exception {
       initFirebaseApp(senderId);
 
       try {
@@ -151,14 +151,18 @@ private String getTokenWithClassFirebaseInstanceId(String senderId) throws IOExc
    }
 
    @WorkerThread
-   private String getTokenWithClassFirebaseMessaging() throws ExecutionException, InterruptedException {
+   private String getTokenWithClassFirebaseMessaging() throws Exception {
       // We use firebaseApp.get(FirebaseMessaging.class) instead of FirebaseMessaging.getInstance()
       //   as the latter uses the default Firebase app. We need to use a custom Firebase app as
       //   the senderId is provided at runtime.
       FirebaseMessaging fcmInstance = firebaseApp.get(FirebaseMessaging.class);
       // FirebaseMessaging.getToken API was introduced in firebase-messaging:21.0.0
       Task<String> tokenTask = fcmInstance.getToken();
-      return Tasks.await(tokenTask);
+      try {
+         return Tasks.await(tokenTask);
+      } catch (ExecutionException e) {
+         throw tokenTask.getException();
+      }
    }
 
    private void initFirebaseApp(String senderId) {