warn instead of throw when resolving keys to model

Before this commit, if you had a payload like:

```json
{
  "posts": []
}
```

But didn’t have a “Post” model defined, Ember
Data would blow up. As discussed in the data
team meeting, we decided to warn if the model
could not be resolved. In this spirit,
Ember Data just doesn’t process what it doesn’t
understand, but can still provide you with a
better debugging message to give you hints on
what your payload should look like.

fixes GH-2337
It feels kind of surprising that a method `_modelExistsForKey` emits a warning. I'd expect it to be a simple check free of side-effects.