Better error message when a `$ref` attribute fails validation

Is your enhancement related to a problem? Please describe.

For the gitlab-ci-schema plus

              ,
              "variables": {
                "markdownDescription": "Additional environment variables that are passed exclusively to the service. The syntax is the same as [Job Variables](https://docs.gitlab.com/ee/ci/variables/index.html).",
                "$ref": "#/definitions/jobVariables"
              }

in services:, input:

test_docker_image_with_service:
  services:
    - name: $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG
      alias: service-under-test
      variables:
        - PORT: ${PORT}

gives:

which a very weird way of saying "please remove that - from inside variables:".

Describe the solution you would like

• Error message should be more clear.
• Also, maybe, "not provide schema-internal information" on the forefront (acceptable on --verbose)

Describe alternatives you have considered

Additional context

Start by reproducing the validation failure for the GitLab CI schema's services.variables $ref using the YAML example in the issue, then trace where that validation error is formatted. Done means the message explains that the list item is invalid and should be removed, while schema-internal details are reserved for verbose output.