Refactor ApiClientError

[zombieobject]

Introduction 📟

ApiClientError has in the past been an enum. However, throwing the enum in an error catching context did not afford the ability to update the localized description.

Purpose ℹ️

Refactor ApiClientError from an Enum to a Struct which conforms to LocalizedError and GenericErrorDescribing. Conforming to LocalizedError enables the ability to updated the localizedDescription and pass along libXMTP API generated errors to SDK clients.

Scope 🔭

• ApiClient
• ApiClientError
• GenericErrorDescribing

Out of Scope ⚔️

• Included tests are out of scope for this pull request. The primary focus of this effort was to forward rate limiting errors to SDK clients. Rate limiting errors are triggered by the ingress point to either the development or production environment; neither of which would be suitable for automated testing.