Add sanity check to archiving script

As discussed offline, we may inadvertently cherry pick a cl to our
release branch without updating the version file, and the bots would
overwrite the original bits for the existing version

This will now raise and exit non zero, making the bot red!

Change-Id: I6b574c7c3d7558ef8cf59de18c9c5e15296d95f2
diff --git a/tools/utils.py b/tools/utils.py
index 4b9600b..90e084b 100644
--- a/tools/utils.py
+++ b/tools/utils.py
@@ -85,6 +85,15 @@
   PrintCmd(cmd)
   subprocess.check_call(cmd)
 
+# Note that gcs is eventually consistent with regards to list operations.
+# This is not a problem in our case, but don't ever use this method
+# for synchronization.
+def cloud_storage_exists(destination):
+  cmd = ['gsutil.py', 'ls', destination]
+  PrintCmd(cmd)
+  exit_code = subprocess.call(cmd)
+  return exit_code == 0
+
 class TempDir(object):
  def __init__(self, prefix=''):
    self._temp_dir = None