Capture and log errors from promise chains

This fixes this problem:

```js
promise.then(function() {
  throw "Fail here for whatever reason"
});
```

The thrown error never sees the light of day because it's treated as
promise rejection and **not** a failure. I encournted this problem when
ED was erroring _internally_ trying to sideload association. The
serializer raised an exception (inside a then callback) that was
swallowed and ruined my god damn day. Here's the offending code:

```js
return this.ajax(this.buildURL(root), "GET",{
      data: this.sinceQuery(since)
    }).then(function(json) {
      // errrors from `didFindAll` are ignored. This is horrible.
      Ember.run(adapter,'didFindAll', store, type, json);
    })
```

EDIT: I want to make something clear. It's these sort of issues that prevent me recommending ember-data to anyone. This seriously crippled @dagda1's and I's brains. The time required to simply figure out what is wrong with JSON parsing is unacceptable. Seriously, who can use this library? /rant.