Exception nao capitura, e fecha a aplicacao

Ola, quando o block catch tenta jogar a exception, o app fecha, e aparece essa mensagem no visual studio. Exception Unhandled Java.Net.UnknownHostException Message=Unable to resolve host "aluracar.herokuapp.com": No address associated with hostname

Coloquei um break point no exception e do catch, e ele mostrou a mesma exception acima.

    try
            {
                using (var client = new HttpClient())
                {
                    var fillForm = new FormUrlEncodedContent(new[]
                    {
                        new KeyValuePair<string,string>("email",login.Email),
                        new KeyValuePair<string,string>("senha",login.Password)
                    });

                    client.BaseAddress = new Uri("https://aluracar.herokuapp.com/login");
                    var result = await client.PostAsync("/login", fillForm);
                    if (result.IsSuccessStatusCode)
                        MessagingCenter.Send<User>(new User(), "SuccessLogin");
                    else
                        MessagingCenter.Send<LoginException>(new LoginException("User or Password Incorrect!"), "FailLogin");
                }
            }
            catch (Exception e) 
            {
                MessagingCenter.Send<LoginException>(new LoginException("Connection Failed! Please check your internet connection and try again later!"), "FailLogin ");
                throw;
            }