feat(acvm)!: Introduce Error type for fallible Backend traits

[phated]

Related issue(s)

Resolves #235

Description

Summary of changes

This adds a required Fallible trait to ACVM backends. It is just used to create an Associated Type called Error that implements std::error::Error because traits aren't allowed to use impl std::error::Error and we don't want to require Backend implementors to have to Box all of their errors.

This uses the Error type to make other trait functions return Result types, which will allow Nargo to handle errors returned by a backend instead of the backend needing to panic. For example, the barretenberg wasm backend and https://github.com/TomAFrench/acvm-backend-groth16 both needed to unwrap/expect a lot of things but those errors should be bubbled up to nargo.

Dependency additions / changes

(If applicable.)

Test additions / changes

(If applicable.)

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.

Additional context

Planning to implement this throughout aztec_backend and nargo.

[TomAFrench]

I'm wondering whether we want to allow for multiple error types here? e.g. ProofSystemCompiler should return different errors to SmartContract?

We could also go more granular.

[phated]

I'm wondering whether we want to allow for multiple error types here? e.g. ProofSystemCompiler should return different errors to SmartContract?

I think it's fine to group them all under one error type. My prototype used a thiserror defined by the backend so it can provide whatever errors it needs.

[phated]

As per @TomAFrench and @kevaundray, I've switched this to having a separate type Error on each trait that has fallible functions.