refactor: introduce `Store(Item)?NotFoundException` classes

[malandis]

This PR detects when a NOT_FOUND gRPC error was specific to the storage client or not. For the storage client NOT_FOUND, there are two cases: the store was not found and the item was not found.

To distinguish these two cases we use string matching on the grpcException message.

[malandis]

Not terribly important or urgent: we may consider organizing the various not found exception classes as follows:

• a base class ResourceNotFoundException, with subclasses
• CacheResourceNotFoundException and StoreResourceNotFoundException
• the latter derives StoreItemNotFoundException and StoreNotFoundException

[malandis]

Naming choices I made:

• StoreNotFoundException for when a store is not found
• StoreItemNotFoundException for when the item is not found. I feel that prefixing with Store reduces ambiguity vs a cache item for users typing in an IDE. Alternatively we could use StorageItemNotFoundException which, while better English, leaves the prefix inconsistent. For grouping purposes a single prefix is a more convenient DX.