dispaly error message if resource could not be found

I had a typo in one of the resource classes. This resulted in an error:

```
jsonapi/routing_ext.rb:53:in `jsonapi_resources': undefined method `routing_resource_options' for nil:NilClass (NoMethodError)
```
I've just started using JR, so this message wasn't very helpful to me. Hope this little patch will save someone some time.