git_cache: print if cache bundle under the same number already exists.

Led-proof: https://chromium-swarm.appspot.com/task?id=4470d83a68228610

R=karenqian

Bug: 943696
Change-Id: Id01e7bae788b52488c536cfc2d93c278d56592fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1585643
Reviewed-by: Karen Qian <karenqian@google.com>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

git_cache.py

@@ -624,11 +624,11 @@ class Mirror(object):
     # might not exist yet, so it will error no matter what
     if ls_err:
       print('Failed to check GS:\n%s' % (ls_err))
-
     if ls_err_ready:
       print('Failed to check GS:\n%s' % (ls_err_ready))
 
     if not (ls_out == '' and ls_out_ready == ''):
+      print('Cache %s already exists' % dest_name)
       return
 
     gsutil.call('-m', 'cp', '-r', src_name, dest_name)