Can we be nicer with this error message? How about suggesting what types we do accept?

```
var msg = format('`fixtures-fs#createFixtures` expected key %j to have value that is a string or object but we received %j', key, value);
```

- Tells user what `key` it was under
- Tells user what we expected as accepted types
- Tells user module name/method it was originating from (could be lost in stacktrace in other cases)