[ServiceWorker] CacheStorage.has should not reject to indicate there is no such cache with cacheName (#781)

Rejection in promises should be an error path but upon the question _has the cache storage a cache named `cacheName`_, possible values are both `true` or `false` so, IMHO, the promise should resolve with _exists_, a boolean value set to `true` if the cache exists or `false` elsewhere.

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/781