Reject promise for null values as an option?

[scotttrinh]

I realize that localForage has taken the stance that looking up a non-existent key should resolve with null instead of reject with an error, but could we make this an optional thing? Maybe a second argument to getItem? I'd be happy to make a PR for it, if everyone likes the idea.

The biggest con is that it wouldn't work reliably if you want to retrieve the value null, but I don't plan on ever doing that in my use case, so... And, if we take the tack of making it a second, optional parameter on getItem, you can still retrieve null if you wanted to.

If not, I'm just going to wrap the $localForage service in my own service, so NBD if we don't want to deviate from localForage, but I thought I'd offer to push this upstream since it's a non-breaking change.