Make bisect not break when buildbucket.get fails.

scripts/slave/recipe_modules/auto_bisect_staging/revision_state.py

Index: scripts/slave/recipe_modules/auto_bisect_staging/revision_state.py
diff --git a/scripts/slave/recipe_modules/auto_bisect_staging/revision_state.py b/scripts/slave/recipe_modules/auto_bisect_staging/revision_state.py
index 39b6640dfc289dac29cac5d52a5b5f7c9182798d..d06a8ac7880345e86c325e548dac24b26d42965d 100644
--- a/scripts/slave/recipe_modules/auto_bisect_staging/revision_state.py
+++ b/scripts/slave/recipe_modules/auto_bisect_staging/revision_state.py
@@ -281,11 +281,15 @@ class RevisionState(object):
 
   def _is_build_failed(self):
     api = self.bisector.api
-    result = api.m.buildbucket.get_build(
-        self.build_id,
-        api.m.service_account.get_json_path(api.SERVICE_ACCOUNT),
-        step_test_data=lambda: api.test_api.buildbot_job_status_mock(
-            api._test_data.get('build_status', {}).get(self.commit_hash, [])))
+    try:
+      result = api.m.buildbucket.get_build(
+          self.build_id,
+          api.m.service_account.get_json_path(api.SERVICE_ACCOUNT),
+          step_test_data=lambda: api.test_api.buildbot_job_status_mock(
+              api._test_data.get('build_status', {}).get(self.commit_hash, [])))
+    except api.m.step.StepFailure:
+      # If the check fails, we cannot assume that the build is failed.
+      return False
     return (result.stdout['build']['status'] == 'COMPLETED' and
             result.stdout['build'].get('result') != 'SUCCESS')