[step_runner] Add a Placeholder Exception step when rendering placeholders raises a StepFailure.

recipes/engine_tests/step_stack_exhaustion.expected/basic.json

Index: recipes/engine_tests/step_stack_exhaustion.expected/basic.json
diff --git a/recipes/engine_tests/step_stack_exhaustion.expected/basic.json b/recipes/engine_tests/step_stack_exhaustion.expected/basic.json
new file mode 100644
index 0000000000000000000000000000000000000000..024aee6abf144629c54956c2cdfaa4baad819275
--- /dev/null
+++ b/recipes/engine_tests/step_stack_exhaustion.expected/basic.json
@@ -0,0 +1,15 @@
+[
+  {
+    "cmd": [
+      "echo",
+      "some",
+      "step"
+    ],
+    "name": "innocent step"
+  },
+  {
+    "name": "$result",
+    "reason": "Uncaught Exception: ValueError('No steps have been run yet, and you are asking for a previous step result.',)",
+    "status_code": -1
+  }
+]