fix: tolerate workflow that needs a missing job (#1595) (#1619)

Change planner functions to return errors

This enables createStages to return `unable to build dependency graph`

Fix PlanEvent to properly report errors relating to events/workflows

pkg/runner/testdata/issue-1595/no-event.yml

@@ -0,0 +1,8 @@
+name: no event
+
+jobs:
+  stuck:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo How did you get here?
+        shell: bash