Add error examples

The errors section needs some examples for clarification. I did not really understand `path` and `links` untile reading https://github.com/json-api/json-api/pull/234#issuecomment-45563232

This PR adds a basic example for an error response omitting `id` and `code`. Main purpose was to make clear how `path` can be used.

I would appreciate if someone else could add another example where a `links` attribute makes sense like trying to update multiple resources at once where one fails or so.