Add methods getOrDefault and getOrThrow

[craigbeck]

Added methods for common patterns found in production code:

Data we know we should have in app:

const value = model.get('_page.user')!;
// following code expects value is defined
// will error in less obvious ways as unlikely to handle the cases of undefined

using getOrThrow

const value = model.getOrThrow('_page.user');
// following code will not be reached
// does not need to defensively handle undefined case

Data we may or may not have but want to cleaner code

const files = model.get('files') || [];

using getOrDefault

const files = model.getOrDefault('files', []);