Reject with indexeddb errors in RemoteConfig.fetch()

[Feiyang1]

await this._storageCache.setLastFetchStatus(lastFetchStatus); in the catch block can fail (e.g. in envs where indexeddb is not available), but the caller can't catch it because we don't catch it and the program just throws.

Refactored the code to use async await directly instead of wrapping fetch inside a Promise callback, so all errors will be caught and rejected properly.

An alternative considered is to try catch in the catch block, then reject, but it makes the code more complicated and less readable.

It fixes #2373 (comment).

[erikeldridge]

Thx, @Feiyang1 !