`eth_getBlockByHash` raises on missing block

Per documentation, https://github.com/ethereum/wiki/wiki/json-rpc#eth_getblockbyhash returns a null object when the block cannot be found - a `Option[Block..]` would be a more natural mapping instead of a an exception with msg `Parameter [result] expected JObject but got JNull` - the latter is hard to filter for, and yet constitutes a "normal" non-exceptional response to the request.