Add error examples

The [errors section](http://jsonapi.org/format/#errors) needs some examples for clarification. I did not really understand `path` and `links` until reading https://github.com/json-api/json-api/pull/234#issuecomment-45563232

This PR adds a basic example for an error response. I am omitting `id` and `code` and `detail`, because the 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.

Wouldn't it make more sense of this to be in the `detail`? I wasn't sure if

> It **SHOULD NOT** change from occurrence to occurrence of the problem, except for purposes of localization

applied on the field or resource level.