Resolves 6731 by adding a custom message for refunding shipping

[Rus0]

Description (*)

Fixed Issues (if relevant)

1. 'Exception' with message 'Warning: Division by zero in vendor/magento/module-sales/Model/Order/Creditmemo/Total/Discount.php on line 32' in vendor/magento/framework/App/ErrorHandler.php:61 #6731: 'Exception' with message 'Warning: Division by zero in vendor/magento/module-sales/Model/Order/Creditmemo/Total/Discount.php on line 32' in vendor/magento/framework/App/ErrorHandler.php:61

Manual testing scenarios (*)

1. Create a order with free shipment
2. Invoice the order
3. Create new creditmemo
4. Refund $10 in Refund Shipping

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-cicd2]

All committers have signed the CLA.

[magento-engcom-team]

Hi @Rus0. Thank you for your contribution
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[magento-engcom-team]

Hi @okorshenko, thank you for the review.
ENGCOM-3294 has been created to process this Pull Request

[magento-engcom-team]

Hi @Rus0. 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.