Move StepFailure and its subclasses to cbuildbot_failures

This CL simply moves a few exception classes from cbuildbot_results.py
to cbuildbot_failures.py, which now hosts all generic exceptions derived
from StepFailure.


BUG=chromium:370979
TEST=`cros lint` + `buildbot/run_tests` pass

Change-Id: If36778798cd21206e1d5d8ef7e46fb649843af27
Reviewed-on: https://chromium-review.googlesource.com/199805
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Yu-Ju Hong <yjhong@chromium.org>
Tested-by: Yu-Ju Hong <yjhong@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index c11b7e9..22f086e 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -24,6 +24,7 @@
 
 from chromite.buildbot import cbuildbot_config
 from chromite.buildbot import cbuildbot_stages as stages
+from chromite.buildbot import cbuildbot_failures as failures_lib
 from chromite.buildbot import cbuildbot_results as results_lib
 from chromite.buildbot import cbuildbot_run
 from chromite.buildbot import constants
@@ -385,7 +386,7 @@
         traceback.print_exc(file=sys.stdout)
         raise
 
-      if not (print_report and isinstance(ex, results_lib.StepFailure)):
+      if not (print_report and isinstance(ex, failures_lib.StepFailure)):
         raise
 
     finally: