Add protection against having out_dir being deleted.
Add protection against having out_dir being deleted.

This can only happen when someone decide to rm -rf run_tha*. Which is silly, but
we don't want run_isolated to crash in that case.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.appspot.com/197900043

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py