title: Swift - Extra argument 'error' in call / nil / call can throw
body: <p>I'm trying to use the core data with swift.
  And I got </p>
  
  <p>Nil is not compatible with expected argument type '()'  and
  Call can throw, but it is not marked with 'try' and the error is not handled</p>
  
  <p>on "context.save(nil)"</p>
  
  <p>and</p>
  
  <p>Extra argument 'error' in call
  on "var result: NSArray = context.executeFetchRequest(request, error:nil)"</p>
  
  <p>what happened?</p>
  
  <pre><code>import UIKit
  import CoreData
  
  class vcMain: UIViewController {
  
      @IBOutlet var txtUsername: UITextField!
      @IBOutlet var txtPassword: UITextField!
  
      @IBAction func btnSave(){
          var appDel: AppDelegate = (UIApplication.sharedApplication().delegate as! AppDelegate)
          var context: NSManagedObjectContext = appDel.managedObjectContext
  
  
          var newUser = NSEntityDescription.insertNewObjectForEntityForName("Users", inManagedObjectContext: context) as NSManagedObject
  
          newUser.setValue("Test Username", forKey: "username")
          newUser.setValue("Test Password", forKey: "password")
  
          print(newUser)
          print("SAVED")
          context.save(nil)
  
      }
  
      @IBAction func btnLoad(){
          var appDel: AppDelegate = (UIApplication.sharedApplication().delegate as! AppDelegate)
          var context: NSManagedObjectContext = appDel.managedObjectContext
  
          var request = NSFetchRequest(entityName: "Users")
          request.returnsObjectsAsFaults = false;
  
          var result: NSArray = context.executeFetchRequest(request, error:nil)
  
  // - (nullable NSArray *)executeFetchRequest:(NSFetchRequest *)request error:(NSError **)error;
  
  
  
      }
  
  
      override func viewDidLoad() {
          super.viewDidLoad()
  
          // Do any additional setup after loading the view.
      }
  
      override func didReceiveMemoryWarning() {
          super.didReceiveMemoryWarning()
          // Dispose of any resources that can be recreated.
      }
  
  }
  </code></pre>