NotFoundException should have a exception message

[enumag]

Many storages (if not all) throw the NotFoundException without any message. It would be better to have a exception message (containing the requested class and key).

Maybe it is not necessary for every storage to throw this exception - when the storage returns an empty value UnitOfWork::reconstititute() will throw the exception anyway and it would be nicer to have it at one place. This change would be a BC Break though.