Rest API handle errors

Hello!

I am trying to get all the errors that I get from my rest API but I think the way that I am doing is wrong, please check my provider and my component

  submit() {
    this.showLoader();
    this.restProvider.customData( this.Data ).then((result) => {
      this.loading.dismiss();
      this.presentSuccessSubmit();
      this.navCtrl.setRoot(HomePage);
    }, (err) => {
      this.loading.dismiss();
      err = JSON.parse(err);
      //console.log(err.data);
      console.log(err);
      this.presentToast(err.data.days,err.data.exc,err.data.hint,err.data.notes);
    });
  } 

Provider

  customData(data) {
    console.log(data);

    var memberid = localStorage.getItem("memberid");
    return new Promise((resolve, reject) => {
        let headers = new Headers();
        headers.append('Content-Type', 'application/json');
        this.http.post(this.apiUrl+'/members/custom/'+memberid, data, {headers: headers})
          .subscribe(res => {
            resolve(res.json());
          }, (err) => {
            reject(JSON.stringify(err.json()));
          });
    });
  }