How to throw an exception based on if else check blocks in flutter dart

I have been trying to implement the exceptions while calling Rest APIs, I have done a basic implementation of exceptions in a try-catch block. But I need to throw error dialogs on different status codes i.e: either 500, 503, 403

I am attaching my code snippets in order to better understand what exactly I am trying to implement.

try {
  var response = await client
      .post(
          Uri.https(
              "api base url", "api end point"),
          body: param)
      .timeout(Duration(seconds: TIME_CONST))
      .catchError(handleError);

  if (response.statusCode == 200) {
    print('Response Body: ${response.body}');
    final data = jsonDecode(response.body);
    return UserCheck(
      user: data['user_exists'],
    );
  } else {
    print("The response status is not 200");
    return param;
  }
} on SocketException {
  throw FetchDataException('message', 'url');
} on TimeoutException {
  throw ApiNotRespondingException("message", "url");
}

Any help will be appreciated. Have been stuck on this problem for quite a few days.