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.

Since `path` links to a single field ("title"), i think the title should be about that specific field.

I understand `detail` as a long form of `title`. So another example could be… a Dog resource where a name field must be one of the [Disney characters](http://simple.wikipedia.org/wiki/List_of_Disney_characters) but the user tried to set it to "steve" (not in that list) instead . Now `title` could say something rather generic like "The dog must be named after a Disney character" whereas `detail` would render a very long error description like "Lorem ipsum Disney […] Goofy, Pluto, Mini Mouse, please […]". Or am i on the wrong track here?