Better hints for JSON serialization errors

When using the shoebox, it would seem that the following would make sense

```js
let response = await fetch(...);
let jsonData = await response.json();
shoeboxStore.currentUser = jsonData;
shoebox.put('user-data', myShoeboxStore);
```
In my case, I ended up getting a cryptic error message (in fastboot only)

Upon investigating this, it turns out that the `jsonData` I am given by way of fetch is likely modified at some point downstream (in my case a `DS.Store#push`), and it then becomes a circular JSON structure as relationships are wired up.

The end result is that, at the time I call `shoebox.put`, my object is serializable, but by the time the page is resolved (and the fastboot `visit` completes), it becomes partially resolved into a much more complex (and circular) structure.

Some ideas for improving this experience:
* Better error messages around shoebox serialization
* Add info to the readme about **when** the shoebox serialization takes place