[r=thumper],[bug=1307290] Fix intermittent destroy error.
[r=thumper],[bug=1307290] Fix intermittent destroy error.

In order to manage the intermittent error that is returned
by os.RemoveAll if the directory is removed by another process
while it is trying, we double check before returning an error.

https://codereview.appspot.com/87500044/
reedobrien/store