Simplify Rust error types

[mat-if]

Summary

Simplify ironfish-rust to single error type and create a small helper for converting to napi errors

The goal here is to make it a little easier to work with errors, spending less time thinking about which type is correct. This also makes it easier to extend functionality (implement From/Into traits, etc).

Testing Plan

Breaking Change

Is this a breaking change? If yes, add notes below on why this is breaking and what additional work is required, if any.

[ ] Yes