Concourse extension (in vscode) doesn't allow `on_error` in Steps

[DJAlPee]

Describe the bug
Concourse extension (in vscode) doesn't allow on_error in Steps, although it's allowed in Jobs. I get the validation error: Unknown property 'on_error' for type 'TaskStep'
Regarding the docs (https://concourse-ci.org/on-error-hook.html and https://concourse-ci.org/jobs.html#schema.job.on_error) on_error is valid for both and it actually works in our environment. It seems, that the fix for the issue #383 is only for the hook in Jobs.

To Reproduce
pipeline.yml

jobs:
- name: myjob
  serial: false
  plan:
    - task: build-app
      params:
        STAGE: dev
        APP: app
      file: build.xml
      on_success:
        put: buildstatus
        params:
          context: build
          repo: src
          state: SUCCESSFUL
      on_error:
        put: buildstatus
        params:
          context: build
          repo: src
          state: FAILED

Sample
The original issue had been fixed with this commit #10d1e620ad15f5add13526341e41a394e6c6bad3. In best case, It's a "quick win" and there is just one line addProp(step, subStep, "on_error", step); to be added in line 424:
https://github.com/spring-projects/sts4/blob/9ef209da74b16a6d28fee3748d55711e5e55a292/headless-services/concourse-language-server/src/main/java/org/springframework/ide/vscode/concourse/PipelineYmlSchema.java#L420-L430

Because my java skills are way outdated and I assume, that some additional test cases should be added here, I would be happy, if someone could take over this one 😉

[kdvolder]

I'm taking a look, this should be an easy fix. My apologies that it took this long to notice this issue. I've been out of working on STS for a while but circling back to it now.