Make the storage system utilize `Identifyable` and typed errors

[twelho]

Currently the storage system still heavily relies on specific ObjectMeta fields (e.g. UID) for identifying objects, refactor the system to use keys that comply to the Idenfityable interface (generating specific unique idenfiers based on some properties of objects) so that the storage can be made oblivious of object details and individual fields. Also wire up typed errors for all operations that can fail.