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

recipes/engine_tests/missing_start_dir.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 deleting the current working directory doesn't immediately fail."""
+
+DEPS = [
+  'step',
+  'path',
+]
+
+
+def RunSteps(api):
+  api.step('innocent step', ['bash', '-c', "echo some step"])
+  api.step('nuke it', ['rm', '-rf', api.path['start_dir']])
+
+  try:
+    api.step('bash needs cwd', ['bash', '-c', "echo fail"])
+    assert True
+  except api.step.StepFailure:  # pragma: no cover
+    assert False
+
+  api.step('python does not', ['python', '-c', 'print "hi"'])
+
+
+def GenTests(api):
+  yield api.test('basic')