[Storage] Small improvements to Jellyfish Merkle Iterator
[Storage] Small improvements to Jellyfish Merkle Iterator

- Rename `visited` to `parent_stack`.
- Replace `unreachable` with returning an error.

aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra