Add AlreadyPosted error for "Status is a duplicate"

We have `AlreadyRetweeted` and `AlreadyFavorited`, but Twitter is also failing when you tweet something twice. This PR adds a specific error class to handle these errors.

We want to use it in our code as such:

```ruby
  def announce video
    Twitter.update message_for_video(video)
  rescue Twitter::Error::AlreadyPosted
    # ignore
  end
```