问如何在Android中使用Retroeft2.0处理无网络连接？

EN

// retrofit, gson
compile 'com.google.code.gson:gson:2.6.2'
compile 'com.squareup.retrofit2:retrofit:2.0.2'
compile 'com.squareup.retrofit2:converter-gson:2.0.2'

public interface ApiInterface {
   @GET("contacts/")
   Call<ContactsModel> getContactsList();
}

public class ApiClient {
public static final String BASE_URL = "http://myexamplebaseurl/";
private static Retrofit retrofit = null;


public static Retrofit getClient() {
    if (retrofit == null) {
        retrofit = new Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .build();
    }
    return retrofit;
  }
}

ApiInterface apiService = ApiClient.getClient().create(ApiInterface.class);

Call<ContactsModel> call = apiService.getContactsList();
    call.enqueue(new Callback<ContactsModel>() {
        @Override
        public void onResponse(Call<ContactsModel> call, Response<ContactsModel> response) {
            if(response.isSuccessful()){
                /*here is my data handling*/
            }

        }

        @Override
        public void onFailure(Call<ContactsModel> call, Throwable t) {
            /*It is the request failure case, 
              I want to differentiate Request timeout, no internet connection and any other reason behind the request failure
            */
        }
    });