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

recipes/engine_tests/step_stack_exhaustion.py

+# Copyright 2017 The LUCI Authors. All rights reserved.
+# Use of this source code is governed under the Apache License, Version 2.0
+# that can be found in the LICENSE file.
+
+"""Tests that step_data can accept multiple specs at once."""
+
+from recipe_engine.util import InputPlaceholder
+from recipe_engine.recipe_api import StepFailure
+
+DEPS = [
+  'step',
+]
+
+class BadPlaceholder(InputPlaceholder):
+  def render(self, test):
+    raise StepFailure("EXPLOSION")
+
+
+def RunSteps(api):
+  try:
+    api.step('innocent step', ['echo', 'some', 'step'])
+
+    ph = BadPlaceholder('name')
+    ph.namespaces = ('fake', 'namespace')
+
+    api.step('bad step', ['echo', ph])
+  finally:
+    api.step.active_result  # this will raise a ValueError
+
+def GenTests(api):
+  yield (
+    api.test('basic') +
+    api.expect_exception('ValueError')
+  )
+