VAT Info: Ensure field validation actually returns an error

[jjchrisdiehl]

As discussed with @sirbrillig, currently the updateError value in the VAT form, under Purchases > Billing History, doesn't return a usable error object to conditionally handle field errors.

We'll need to investigate this further and find out why we aren't receiving a usable error.

Related to #100515

[github-actions]

OpenAI suggested the following labels for this issue:

• [Feature Group] WPCOM Store & Purchases: The issue pertains to the VAT form and billing history, which are directly related to purchases and billing on WordPress.com.
• [Feature] Checkout: This label is relevant as it involves ensuring that errors related to the VAT form entry (which is part of the checkout process) are validated properly.
• [Feature] Purchase Management: This is applicable because the issue addresses the handling of errors within the purchase process related to the VAT field.

[sirbrillig]

Thanks for making an issue. I believe the issue is that useMutation requires you to specify when a request has an error, and wp.req.get() probably does not fail for validation failures. If we want the validation failure to set the error property it returns, I think we need to notice such a failure and explicitly set the error (probably by throwing).