javascript - Promises with React

I have questions about promises. I'm just starting to deal with them and it's not that easy to understand!

I'm trying to setup an authentication system for my app.

RegisterPage

handleSubmit looks like that:

handleSubmit(event) {
  event.preventDefault();     
  const { user } = this.state;
  //some code here
  userActions.register(user);

}

UserActions

function register(user) {
  userService.register(user)
    .then(
      user => {
        success(user);

      },
      error => {
        failure(error.toString());
      }
    );
  function success(user) { return { type: "REGISTER_SUCCESS", user } }
  function failure(error) { return { type: "REGISTER_ERROR", error } }
}

UserService

function register(user) {
  const requestOptions = {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify(user)
  };

  return fetch(`/api/users/register`, requestOptions).then(handleResponse);

}

function handleResponse(response) {
  return response.text().then(text => {
      const data = text && JSON.parse(text);
      if (!response.ok) {  
          const error = (data && data.message) || response.statusText;
          return Promise.reject(error);
      }


      return data;
  });
}

Question 1. That code is "working" but not like I want. That way, even if the request success, I still can have error from the server, like duplicate username or something like that. I guess what I want is to return Promise.reject() not just if !response.ok but also if I have errors in the JSON returned right?

function handleResponse(response) {
  return response.text().then(text => {
      const data = text && JSON.parse(text);

      if (!response.ok) {  
          const error = (data && data.message) || response.statusText;
          return Promise.reject(error);
      }
      else if(data.errors) {
          return Promise.reject(data.message);
      }

      return data;
  });

}

Question 2. If everything's fine, should I return data or return Promise.resolve(data)? And why?