Pipeline succeeds if setup step fails

If the first step of a pipeline fails (e.g. because the container couldn't be started), the whole pipeline is marked as successful when it shouldn't be. See https://buildkite.com/bazel/bazel-bazel-github-presubmit/builds/5453 as an example of that happening.

Start by reviewing the linked Buildkite build 5453 and trace how pipeline setup-step failures are reported. The fix is complete when a failed initial setup step causes the entire pipeline to be marked unsuccessful, including container-start failures.

Written by the indexing model from the issue text.