Provide option to downgrade `unknown import` errors to warnings

When instantiating a component, it's probably safe to supply it with additional imports that aren't declared in its world. For example, consider a component that declares an import `foo` in its world but it doesn't use `foo` in its implementation. Consequently, `foo` won't be present in the component's WIT that is generated by `witgen`, which will cause a confusing `unknown import` error by `wac`. I think downgrading the error to a warning would be enough to let the user know about a potential logical error while allowing `wac` to run successfully.