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

recipe_engine/step_runner.py

 # Copyright 2016 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.
 
 import calendar
 import collections
 import contextlib
 import datetime
 import itertools
 import json
@@ skipping 266 matching lines @@
     return getattr(value, '__name__', 'UNKNOWN_CALLABLE')+'(...)'
 
   def _print_step(self, step_stream, step, env):
     """Prints the step command and relevant metadata.
 
     Intended to be similar to the information that Buildbot prints at the
     beginning of each non-annotator step.
     """
     def gen_step_prelude():
       yield ' '.join(map(_shell_quote, step.config.cmd))
-      yield 'in dir %s:' % (step.config.cwd or os.getcwd())
+      cwd = step.config.cwd
+      if cwd is None:
+        try:
+          cwd = os.getcwd()
+        except OSError as ex:
+          cwd = '??? (ENGINE START_DIR IS MISSING: %r)' % (ex,)
+      elif not os.path.isdir(cwd):
+          cwd += ' (ENGINE START_DIR IS MISSING OR NOT A DIR)'
+      yield 'in dir %s:' % (cwd,)
       for key, value in sorted(step.config._asdict().items()):
         if value is not None:
           yield ' %s: %s' % (key, self._render_step_value(value))
       yield 'full environment:'
       for key, value in sorted(env.items()):
         yield ' %s: %s' % (key, value)
       yield ''
     stream.output_iter(step_stream, gen_step_prelude())
 
   def _run_cmd(self, cmd, timeout, handles, env, cwd):
@@ skipping 442 matching lines @@
   supplied command, and only uses the |env| kwarg for modifying the environment
   of the child process.
   """
   saved_path = os.environ['PATH']
   try:
     if path is not None:
       os.environ['PATH'] = path
     yield
   finally:
     os.environ['PATH'] = saved_path