[step_runner] do not immediately bail when START_DIR is gone.

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."""
+"""Tests that placeholders can't wreck the world by exhausting the step stack."""
 
 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'])
+    api.step('innocent step', ['bash', '-c', "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')
   )