Expose error code and httpStatusCode on `.checkoutUnavailable` error

[markmur]

What changes are you making?

Expose error code and httpStatusCode on .checkoutUnavailable error

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Before you merge

[!IMPORTANT]

• [ ] I've added tests to support my implementation
• [x] I have read and agree with the Contribution Guidelines.
• [x] I have read and agree with the Code of Conduct.
• [ ] I've updated the README.

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Checklist for releasing a new version

- [ ] I have bumped the version number in the [`podspec` file](https://github.com/Shopify/checkout-kit-swift/blob/main/ShopifyCheckoutKit.podspec#L2). - [ ] I have bumped the version number in the [README](https://github.com/Shopify/checkout-kit-swift/blob/main/README.md#packageswift). - [ ] I have added a [Changelog](./CHANGELOG.md) entry.

[!TIP] See the Contributing documentation for instructions on how to publish a new version of the library.