Fix md5sum.py crashing when trying to delete non-existent md5sum directory

Fix md5sum.py crashing when trying to delete non-existent md5sum directory

"rm -rf foo/" fails, but "rm -rf foo" succeeds

BUG=

Committed: https://crrev.com/a9d40e8aedcad7433eed48f3437f39f0ccfc2a86
Cr-Commit-Position: refs/heads/master@{#348415}

[agrieve, 2015-09-11 13:58:20]

agrieve@chromium.org changed reviewers:
+ perezju@chromium.org

[jbudorick, 2015-09-11 14:02:34]

jbudorick@chromium.org changed reviewers:
+ jbudorick@chromium.org

[jbudorick, 2015-09-11 14:02:35]

lgtm

[agrieve, 2015-09-11 15:44:34]

The CQ bit was checked by agrieve@chromium.org

[commit-bot: I haz the power, 2015-09-11 15:45:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1332373002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1332373002/1

[commit-bot: I haz the power, 2015-09-11 16:54:14]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-11 16:54:45]

Patchset 1 (id:??) landed as
https://crrev.com/a9d40e8aedcad7433eed48f3437f39f0ccfc2a86
Cr-Commit-Position: refs/heads/master@{#348415}

[commit-bot: I haz the power, 2015-09-23 12:23:16]

Patchset 1 (id:??) landed as
https://crrev.com/a9d40e8aedcad7433eed48f3437f39f0ccfc2a86
Cr-Commit-Position: refs/heads/master@{#348415}