Exceptions on the cache-store should be ignored for Read/WriteThroughStore

I'm not sure that those behaviors are intended or not.
But using `Future#respond` variants like `.onFailure { case x: Exception => storeValue }` are meaningless.
(`onFailure` won't transform into new `Future`, it will simply register a callback)

Use `Future#transform` variants instead of `Future#respond`s on XxxThroughStore