fix(core/schema): set explicit enumerability on error.$response

[kuhe]

I noticed that if you define a property as undefined and then call Object.defineProperty on it with only a value, it no longer default to non-enumerable.

The ServiceException (base class) from @smithy/client has:

export class ServiceException extends Error implements SmithyException, MetadataBearer {
  readonly $fault: "client" | "server";
  $response?: HttpResponse;

which creates a class instance level key that is undefined.