const customIntervalObservable = new Observable(observer => {
     let count = 0;
      setInterval( () => {
        //We can call the next method to emit a new value.
        observer.next(count);
        count++;
        if(count > 3) {
          observer.error(new Error('Count is greater 3!'));
        }
      }, 1000);
    });

    this.firstObsSubscription = customIntervalObservable.subscribe(data => {
      console.log(data);
    }, error => {
      // to handler the error we need to pass another argument to subscribe
      console.log(error.message);
    });
  }