Make sure that OpenAPI rules violations errors show up in Prow build logs
Make sure that OpenAPI rules violations errors show up in Prow build logs

I noticed recently that `pull-tekton-pipeline-build-tests` will properly fail for a new OpenAPI rules violation, but the failure doesn't get shown on the Prow page for the job unless you expand to see the full log. This is because the error message doesn't contain any of the strings Spyglass/Prow are searching for in the logs to identify errors/failures. So let's add `ERROR:` to the message.

Signed-off-by: Andrew Bayer <584a3be2b8e7a970b37393618731e9be5df24fde@gmail.com>

tektoncd/pipeline,tektoncd/pipeline