Use a jsonschema error parser better suited to GitHub Actions

[CodeGat]

Currently, schema validation errors in json are reported in json itself. This is fine if you know what you're looking for, but otherwise it difficult to ascertain what is wrong with the data.

See if there exist any better jsonschema error parsers on the GitHub Actions Marketplace, or see if the output of GrantBirki/json-yaml-validate can be a bit more human readable.