fix(cardano-chain-follower): set_read_pointer now returns None when the requested point is not found

[FelipeRosa]

Description

This PR fixes a bug I found while testing the Cardano Runtime Extension.

Calling Follower::set_read_pointer should return Some(point) if the requested point was found on chain (and the follower is now following from that point) and None if the point was not found. The current implementation always returns None.

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module

[cong-or]

lgtm