Clarify production setup ref validation

Author: justin808

bin/test-cpflow-github-flow

@@ -44,6 +44,7 @@ require "yaml"
 
 CONTROL_PLANE_FLOW_WORKFLOW = %r{\Ashakacode/control-plane-flow/\.github/workflows/[^@\s]+@([^\s]+)\z}
 PROMOTE_WORKFLOW = %r{\Ashakacode/control-plane-flow/\.github/workflows/cpflow-promote-staging-to-production\.yml@([^\s]+)\z}
+EXPECTED_PROMOTE_WORKFLOW_REF_FORMAT = "shakacode/control-plane-flow/.github/workflows/cpflow-promote-staging-to-production.yml@vX.Y.Z"
 EXPECTED_CPFLOW_CHECKOUT_ACTION = "actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd"
 EXPECTED_CPFLOW_CHECKOUT_REPOSITORY = "shakacode/control-plane-flow"
 
@@ -130,7 +131,8 @@ setup_ref = setup_step.fetch("with", {})["control_plane_flow_ref"]
 setup_match = setup_ref.to_s.match(PROMOTE_WORKFLOW)
 
 unless setup_match
-  abort "#{promote_path}:promote-to-production must pass a pinned production control_plane_flow_ref to setup"
+  abort "#{promote_path}:promote-to-production must pass a pinned production control_plane_flow_ref to setup, " \
+        "for example #{EXPECTED_PROMOTE_WORKFLOW_REF_FORMAT}"
 end
 
 refs[setup_match[1]] << "#{promote_path}:promote-to-production setup"