DiscountCode REST resource can't handle error responses

Issue summary

If Shopify responds with a 4xx or 5xx status code while creating a discount code via the REST API, then a NoMethodError is raised.

• shopify_api version: 13.3.1
• Ruby version: 3.2.2
• Operating system: Ubuntu 22.04 LTS

Expected behavior

1. The DiscountCode resource should not set @errors to nil on initialization. It should leave it set from the base class as an instance of Rest::BaseErrors
2. When a ShopifyAPI::Errors::HttpResponseError is encountered during a save, the error is appended onto @errors.errors and the exception is re-raised

Actual behavior

• In the ShopifyAPI::Rest::Base class, an @errors variable is initialized
• In all versions of the ShopifyAPI::DiscountCode resource, the initializer resets @errors to nil
• If a ShopifyAPI::Errors::HttpResponseError is encountered while saving, we append the error onto @errors.errors
• Since @errors is nil on the DiscountCode resource, we get the following:

NoMethodError: undefined method `errors' for nil:NilClass

Steps to reproduce the problem

Add the following test case to test/rest/2023_10/discount_code_test.rb and run it. It will blow up with the no method error I described:

  sig do
    void
  end
  def test_10()
    stub_request(:post, "https://test-shop.myshopify.io/admin/api/2023-10/price_rules/507328175/discount_codes.json")
      .to_return(status: 500, body: JSON.generate({"errors":[{"message":"There was a problem loading this website. Please try again.","extensions":{"code":"SERVICE_UNAVAILABLE"}}],"error_reference":"If you report this error, please include this id: 9e6928cf-99cd-47e0-a257-b335115453dc."}), headers: {})

    response = discount_code = ShopifyAPI::DiscountCode.new
    discount_code.price_rule_id = 507328175
    discount_code.code = "SUMMERSALE10OFF"
    discount_code.save!
  end

Start with lib/shopify_api/rest/base.rb, the DiscountCode resource initializer under lib/shopify_api/rest/resources/2023_10/discount_code.rb, and test/rest/2023_10/discount_code_test.rb. Run the provided failing 500-response scenario, inspect how save! records HttpResponseError instances, and confirm DiscountCode retains Rest::BaseErrors and re-raises while the test passes.