Rework error handling

[kevin-harrison]

Right now Omnipaxo's functions panic on errors instead of returning results. This makes it awkward for users to recover their crashed Omnipaxos instances. It's standard for library crates to generate thiserror errors and allow users to handle them. Furthermore, Omnipaxos can be made to recover from errors in the back-end storage without restarting the entire instance. We need to be careful to only update in-memory state if a call to the back-end storage was successful.