message: Raise an error first if `errors` are present and second if `data` is missing.
  Ref:
diff: @@ -117,14 +117,14 @@ export const fetchSchema = async (
  })
  );
  - if (!data) {
  - throw new Error("No data received from server introspection.");
  - }
  -
  if (errors) {
  throw new Error(errors.map(({ message }: Error) => message).join("\n"));
  }
  + if (!data) {
  + throw new Error("No data received from server introspection.");
  + }
  +
  return data.__schema;
  };
  