Add `warn_on_failure` option on CI builds
When enabled, when this build fails, the pipeline will be successful/green for all intents and purposes, but a warning will be displayed on the MR or commit or build page.
This is to be used by builds that are allowed to fail, but where failure indicates some other (manual) steps should be taken elsewhere. allow_failure
builds, on the other hand, just fail silently and are easily ignored or completely forgotten about.