Adding delegate method to tell that we found access error to photos.

Hi,

I've added two more `WSAssetPickingState` to show error and deliver message to presenting controller about the error. 

When user doesn't allow app to access photos, `WSAlbumTableViewController` keeps showing `@"Loading…"`. So I've added `WSAssetPickerStateAccessError` to deliver message to delegate that it has meet `ALAssetsLibraryAccessUserDeniedError`. There's another generic error to cover other kind of errors.

![screenshot](https://cloud.githubusercontent.com/assets/856233/2946059/d988e160-d9e7-11e3-8be4-7900dfd1da40.png)