Json响应错误处理

Question

我的应用程序有一个搜索栏，可以向MySQL数据库发送查询，然后接收JSON响应并在视图中显示它。当响应成功时（MySQL中存在实际的数组和行），这很有用。但是，当响应为空时，我想显示一个带有简单错误的警告框。

当查询响应为空时，我的服务器上的PHP代码设置为echo No rows，如何在我的应用程序中实现此目的？

这就是我现在要做的事情：

    func searchBarSearchButtonClicked(searchBar: UISearchBar)
{
    if(searchBar.text!.isEmpty)
    {
        return
    }

    doSearch(searchBar.text!)

}

func doSearch(searchWord: String)
{
    mySearchBar.resignFirstResponder()
    //created NSURL
    if let requestURL = NSURL(string: URL_GET_coffees) {
        //creating NSMutableURLRequest
        let request = NSMutableURLRequest(URL: requestURL)
        //setting the method to post
        request.HTTPMethod = "POST"
        //getting values from search bar text field
        let searchText=mySearchBar.text
        //creating the post parameter from search bar text field
        let postParameters = "searchQuery="+searchText!;
        //adding the parameters to request body
        request.HTTPBody = postParameters.dataUsingEncoding(NSUTF8StringEncoding)
        //creating a task to send the post request
        let task = NSURLSession.sharedSession().dataTaskWithRequest(request){
            data, response, error in
            //exiting if there is some error
            if error != nil{
                print("error is \(error)")
                self.displayAlertMessage(error!.localizedDescription)
                return;
            }
            //parsing the response
            do {
                guard let coffees = try NSJSONSerialization.JSONObjectWithData(data!, options: .MutableContainers) as? NSArray else {
                    //Doesn't exist, or isn't an NSArray
                    return
                }

                var newcoffees=[face]()

                for coffee in coffees {
                    //getting the data at each index
                    let coffeeName = coffee["name"] as! String
                    let coffeeDirectLink = coffee["calculation_toemail_"] as! String
                    let coffeeImage = coffee["calculation_toemail_"] as! String

                    let newface = face(name:coffeeName,
                                       directLink: coffeeDirectLink,
                                       image:coffeeImage)
                    newcoffees.append(newface)
                    //displaying the data
                    print("name -> ", coffeeName)
                    print("direct_link -> ", coffeeDirectLink)
                    print("image -> ", coffeeImage)
                    print("===================")
                    print()
                }
                dispatch_async(dispatch_get_main_queue(),{
                    self.faces = newcoffees
                    self.collectionview.reloadData()
                })

                 let errorMessage = newcoffees["No rows"] as? String
                if(errorMessage != nil)
                {
                   // display an alert message
                    self.displayAlertMessage(errorMessage!)
                }

            } catch {
                print(error)
            }
        }
        //executing the task
        task.resume()
    }
}

func displayAlertMessage(userMessage: String)
{
    let myAlert = UIAlertController(title: "Alert", message: userMessage, preferredStyle: UIAlertControllerStyle.Alert);
    let okAction =  UIAlertAction(title: "Ok", style: UIAlertActionStyle.Default, handler: nil)
    myAlert.addAction(okAction);
    self.presentViewController(myAlert, animated: true, completion: nil)
}

但是发生的事情是我得到一个Cannot subscript a value of type '[face]' with an index of type 'String'，我告诉它错误响应是什么。我能做错什么？