Fixes cart item validation and error handling on checkout

[kidunot89]

Your checklist for this pull request

Thanks for sending a pull request! Please make sure you click the link above to view the contribution guidelines, then fill out the blanks below.

🚨Please review the guidelines for contributing to this repository.

• Make sure you are making a pull request against the develop branch (left side). Also you should start your branch off our develop.
• Make sure you are requesting to pull request from a topic/feature/bugfix branch (right side). Don't pull request from your master!

What does this implement/fix? Explain your changes.

• Processes cart item validation errors during the checkout mutation.
• Removes deprecated fields and argument forgotten in v0.5.x release.
• Adds support for changes made to WC_Cart::add_to_cart().

Does this close any currently open issues?

Fixes #313
Fixes #308

Any relevant logs, error output, GraphiQL screenshots, etc?

(If it’s long, please paste to https://ghostbin.com/ and insert the link here.)

Any other comments?

…

Where has this been tested?

Operating System: Ubuntu 20.04

WordPress Version: 5.4.2