Chore(host, client, mpt): Custom error types

[nhtyy]

Remove all uses of eyre/anyhow from the lib, and remove any panics in fns that return results

[yuwen01]

I think eyre is the appropriate choice for errors in RSP, since its primary use is as a standalone binary, not a library. For example, we're never going to publish RSP on crates.io. Open to being convinced though.

I agree with getting rid of the MPT panics -- is there a way to use eyre instead of thiserror to better handle the error?

[nhtyy]

ooo interesting.. I think in MPT I like thiserror better because it shows the discrete set of possible errors (regardless of if therye recoverable or not) but using eyre doesn't relay such information.

I have a (maybe contrived) use case actually for exposing RSP as a lib: you can use RSP to augment light client sync protocols to get execution validity proofs