title: Swift Promise Kit and throws
body: <p>So I'm using PromiseKit in my latest swift app to do most of the networking code, along with Alamofire. I'm trying to setup my promises to throw when my returns aren't what I desire - here's what the code looks like:</p>
  
  <p>`</p>
  
  <pre><code>    do{
          firstly({
              try DoStuff.doStuff()
          }).then({ response in
              self.array = response
          }).error { error in
              throw Error.GeneralError
              print(error)
          }
  
          firstly({
              try DoOtherThing.otherThing()
          }).then({ response in
              self.stuff = response
          }).error{ error in
              throw TransactionError.GeneralError
              print(error)
          }
      } catch {
          let alertController = UIAlertController(title: "Network Error", message: "Network error, please try again", preferredStyle: .Alert)
          let OKAction = UIAlertAction(title: "OK", style: .Default) { (action) in
              //
          }
          alertController.addAction(OKAction)
          self.presentViewController(alertController, animated: true) {
              //
          }
      }
  </code></pre>
  
  <p>`</p>
  
  <p>This code works just hunky dory if I don't have the 'throw' statements in there - if I just print the error, or put my alert controller code in there, works as expected. But when I add the throw, I get an compiler red flag on the 'error' line that says <code>Cannot call value of non function type 'ErrorType'</code> Any thoughts? Thanks</p>