Test coverage for PR-226

[naydav]

Need to create API-functional tests for
#226

Also, need to cover all of the negative scenarios (see all of Exceptions in PR)

Examples of tests: dev/tests/api-functional/testsuite/Magento/GraphQl/*
Documentation: https://magento-devdocs.github.io/devdocs-for-tests/guides/v2.3/graphql/functional-testing.html

[naydav]

#347