Update the return type in the tax rate ajax controllers to be correct

[dmanners]

Description

While working on the Json controllers task I notices a couple of the Ajax controllers in Magento Tax had the wrong or no return type or did not show that they actually could throw an exception with the call to $this->resultFactory->create

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)