Fix throwing error by checkout error processor model

[ihor-sviziev]

Description

See #18330 for steps to reproduce and additional details

Fixed Issues (if relevant)

1. Checkout - Infinite loading indicator when server returned error #18330: Checkout - Infinite loading indicator when server returned error

Manual testing scenarios

See #18330

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @ihor-sviziev. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
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[magento-engcom-team]

Hi @VladimirZaets, thank you for the review.
ENGCOM-3073 has been created to process this Pull Request

[magento-engcom-team]

Hi @ihor-sviziev. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.