Better hints for JSON serialization errors

[mike-north]

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

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