[STORE] Don't catch FNF/NSF exception when reading metadata

When reading metadata we do catch FileNotFound and NoSuchFileExceptions
today, log the even and return an empty metadata object. Yet, in some cases
this might be the wrong thing todo ie. if a commit point is provided these
situations are actually an error and should be rethrown. This commit
pushes the responsiblity to the caller to handle this exception.
I wonder why this change? initial value is unused and once existing files are set, they are never changed.