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/deploy_chrome.py b/scripts/deploy_chrome.py
index 07db833..21716ab 100644
--- a/scripts/deploy_chrome.py
+++ b/scripts/deploy_chrome.py
@@ -33,7 +33,7 @@
 
 
 from chromite.buildbot import constants
-from chromite.buildbot import cbuildbot_results as results_lib
+from chromite.buildbot import cbuildbot_failures as failures_lib
 from chromite.cros.commands import cros_chrome_sdk
 from chromite.lib import chrome_util
 from chromite.lib import cros_build_lib
@@ -77,7 +77,7 @@
   return url.rstrip('/').rpartition('/')[-1]
 
 
-class DeployFailure(results_lib.StepFailure):
+class DeployFailure(failures_lib.StepFailure):
   """Raised whenever the deploy fails."""
 
 
@@ -712,5 +712,5 @@
       deploy = DeployChrome(options, tempdir, staging_dir)
       try:
         deploy.Perform()
-      except results_lib.StepFailure as ex:
+      except failures_lib.StepFailure as ex:
         raise SystemExit(str(ex).strip())