.match should reject if the request wasn't in the cache

I checked with [the spec](https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#cache-match) and it seems that `.match` should reject with a `NotFoundError` if there was no match in the cache.

`NotFoundError` is a `DOMException` type, which can't actually be constructed in user code, so I'm just using `Error` and including `NotFoundError` in the message.