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', []);

[colincollerlever]

Is there a way we can include the collection name in the error? I don't know if it's really needed, if we get the call stack, but it might be helpful.

[ericyhwang]

The full path will already include the collection name - Model#_at is the child model's absolute path from the root.

[ericyhwang]

Mentioning for discussion - as this is now, it won't support a 1-arg call like getOrDefault(defaultValue), though there is the workaround of doing getOrDefault('', defaultValue).

We can always add it later if needed and we don't want to add it now.

[adin-ttc]

looks like the intent is as a fallback in case the subpath fails? . which would make sense then to keep the structure... I'd argue that providing blank subpath or defaultValue raise error no? They are both required?

[craigbeck]

Eric and I discussed ofline and agree the no-subpath case is unlikely, so leaving as is and see if any use comes up.