Twitter::Error.errors lists descendants recursively

While <del>stealing</del> borrowing the improved client/server exception handling approach for Octokit.rb, I noticed that `Twitter::Error.errors` throws an exception:

```
>> Twitter::Error.errors
NameError: uninitialized constant Twitter::Error::ClientError::HTTP_STATUS_CODE
        from /Users/wynn/.coral/repos/twitter@sferik/lib/twitter/error.rb:20:in `block in errors'
        from /Users/wynn/.coral/repos/twitter@sferik/lib/twitter/error.rb:19:in `each'
...
```
This change makes that method a bit more defensive and allows child error types to be listed recursively.