[CORE] Don't handle FNF exceptions when reading snapshot

We used to handle FNF exceptions in the store when reading a snapshot.
For instance if we can't open a segments file for a given commit point
we just return an empty metadata object and tracelog the even. This can
cause shards to be false marked as corrupted if a shard is forcefully
removed while a recovery started at the same time. We should in general
bubble up these exceptions and let the caller decided how to handle the
IOExceptions.