Add error handling for "Twitter profile not found", "Twitter appears to be down" and "Twitter is busy"

[ccharles]

From Issue #2:

• If the Twitter ID doesn't exist, the nametag is still created but is empty.

  A simple test for this could be a null length object from the getJSON call

• Twitter.com isn't tested to see if it is available. We need to ensure Twitter is responding and throw a friendly error for humans if it isn't.

  Let's start by testing for a non-200 HTTP Response Code from the tweeturl variable The jsXHR object looks useful here for response codes and hopefully a null state as well: http://api.jquery.com/jQuery.ajax/#jqXHR

[ccharles]

Fixed in 75d3739d8a94ff6ce2d7ba6f0f236628782b0959