Revert "storage: FS.GetManifest: Also check other files; beside CURRENT file"

This reverts commit fa9b7158e0d220aaae2b9710a45273865509481f.

Better implement this at the leveldb level instead of at storage level.

Related to issue #49.
leveldb: Fix DB that get trashed if ErrorIfMissing is false and manifest file is missing

Should also check for other files to detect presence of old DB.

Should fix issue #49.