Improve CI Linter error message for a job without a script attribute

Previously a .gitlab-ci.yml worked fine like this:

before_script:
  - echo 'test'

test:
  image: ruby:2.3

But a recent change made it required for a job to be defined with a script attribute, so this now fails the linter. It now needs to look like this:

before_script:
  - echo 'test'

test:
  image: ruby:2.3
  script: echo 'test2'

Entering the first YML file only says Status: syntax is incorrect and Error: Undefined error, nothing else. This should be more informative.