Bug that allows builds to be started on non-existent stages

[yakanechi]

What happened: There is no error, e.g. with validator, even if there is a job with a non-existent stage specified as requires.

Therefore, the build can be started. However, no jobs are displayed in the UI.

What you expected to happen: Errors occur in jobs where a non-existent stage is specified in validator (config-parser).

How to reproduce it: https://cd.screwdriver.cd/pipelines/15710/events/830613

jobs:
  ci-deploy:
    requires: []
  ci-test:
    requires: [ci-deploy]
  ci-certify:
    requires: [ci-test]
  triggered-by-stage:
    requires: [~stage@integration2] # <- Caused by

stages:
  integration:
    requires: [~commit]
    jobs: [ci-deploy, ci-test, ci-certify]
    description: "This stage will deploy the latest application to the CI environment and certify it after the tests have passed."

[minghay]

@yakanechi It looks like the validator does not validate that any of the requires jobs exist:

jobs:
    foo:
        requires: [bar]

For remote join jobs, I think it is ok not validating that those jobs exist; however, the validator should be updated to verify that any local jobs/stages referenced within the configuration exist. You should be able to use the existing code that the pipeline stages uses for validating the jobs array configuration.