Add Products Error Widget

class _ProductsErrorWidget extends StatelessWidget {
  final String error;

  const _ProductsErrorWidget({Key key, @required this.error})
      : assert(error != null),
        super(key: key);

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      crossAxisAlignment: CrossAxisAlignment.stretch,
      children: [
        Text(
          error,
          textAlign: TextAlign.center,
          style: Theme.of(context).textTheme.headline6,
        ),
        const SizedBox(height: 16),
        Align(
          child: RaisedButton(
            onPressed: context.read<ProductsCubit>().refresh,
            child: const Text('Retry'),
          ),
        ),
      ],
    );
  }
}