Improve error message

`fixture` is an instance of `Object`.

Before:
```
the id property must be defined for fixture [Object object]
```

After:
```
the id property must be defined for fixture {"foo": "bar"}
```
Use `toString` if environment doesn't support `JSON.stringify`
Update to 'relationship' terminology
Remove unused var declaration
Merge branch 'master' of https://github.com/emberjs/data into terminology-cleanup
Merge pull request #810 from bradleypriest/patch-4

Remove unused var declaration
Merge pull request #809 from JasonKriss/terminology-cleanup

Update to 'relationship' terminology
Merge pull request #805 from tricknotes/improve-error-message

Improve error message