Gsutil ls misses should not be treated as failures by bisect.

scripts/slave/recipe_modules/auto_bisect/api.py

Index: scripts/slave/recipe_modules/auto_bisect/api.py
diff --git a/scripts/slave/recipe_modules/auto_bisect/api.py b/scripts/slave/recipe_modules/auto_bisect/api.py
index f77ca715e59a2093a9da2b8594207623932c1189..d914e5ce14bf525608f31503beed36229d5e3b9d 100644
--- a/scripts/slave/recipe_modules/auto_bisect/api.py
+++ b/scripts/slave/recipe_modules/auto_bisect/api.py
@@ -99,6 +99,9 @@ class AutoBisectApi(recipe_api.RecipeApi):
     try:
       self.m.gsutil(['ls', path])
     except self.m.step.StepFailure:  # pragma: no cover
+      # A step failure here simply means that the file does not exist, and
+      # should not be treated as an error.
+      self.m.step.active_result.presentation.status = self.m.step.SUCCESS
       return False
     return True