[engine] make Uncaught Exception steps purple.

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 161 matching lines @@
 
       return EmptyOpenStep()
 
     try:
       rendered_step = render_step(
           step_config, recipe_test_api.DisabledTestData()
       )
       step_config = None  # Make sure we use rendered step config.
     except:
       with self.stream_engine.make_step_stream('Placeholder Exception') as s:
+        s.set_step_status('EXCEPTION')
         with s.new_log_stream('exception') as l:
           l.write_split(traceback.format_exc())
       raise
 
     step_env = _merge_envs(os.environ, (rendered_step.config.env or {}))
     # Now that the step's environment is all sorted, evaluate PATH on windows
     # to find the actual intended executable.
     rendered_step = _hunt_path(rendered_step, step_env)
     self._print_step(step_stream, rendered_step, step_env)
 
@@ skipping 225 matching lines @@
 
   def open_step(self, step_config):
     try:
       test_data_fn = step_config.step_test_data or recipe_test_api.StepTestData
       step_test = self._test_data.pop_step_test_data(step_config.name,
                                                      test_data_fn)
       rendered_step = render_step(step_config, step_test)
       step_config = None  # Make sure we use rendered step config.
     except:
       with self.stream_engine.make_step_stream('Placeholder Exception') as s:
+        s.set_step_status('EXCEPTION')
         with s.new_log_stream('exception') as l:
           l.write_split(traceback.format_exc())
       raise
 
     # Layer the simulation step on top of the given stream engine.
     step_stream = self._stream_engine.new_step_stream(rendered_step.config)
 
     class ReturnOpenStep(OpenStep):
       def run(inner):
         timeout = rendered_step.config.timeout
@@ skipping 301 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