Fixed bug in story_runner's has_existing_exception.

Fixed bug in story_runner's has_existing_exception.

sys.exc_info() returns a tuple of (None, None, None) to indicate that
no exception is currently being handled. Comparing this against None
always returns true, so exceptions thrown in DidRunStory are silently
being squelched.

This change has been tested with some forthcoming tests, but otherwise
there's no test coverage for this fix yet. (There's also no test
coverage of the existing code path, hence the bug.)

BUG=495870
Committed: https://crrev.com/86984d5fc56ce00e7b37ebe798aefa1d5374c83e
Cr-Commit-Position: refs/heads/master@{#339971}

[Ken Russell (switch to Gerrit), 2015-07-22 20:13:25]

kbr@chromium.org changed reviewers:
+ aiolos@chromium.org, eakuefner@chromium.org, nednguyen@google.com

[Ken Russell (switch to Gerrit), 2015-07-22 20:13:25]

Please review. I'd like to split this fix off from my forthcoming CL since the
latter will probably take a lot longer to review. Thanks.

[aiolos (Not reviewing), 2015-07-22 20:16:58]

lgtm

[aiolos (Not reviewing), 2015-07-22 20:19:44]

The CQ bit was checked by aiolos@chromium.org

[commit-bot: I haz the power, 2015-07-22 20:19:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1250123002/1

[commit-bot: I haz the power, 2015-07-22 23:06:20]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-22 23:06:56]

Patchset 1 (id:??) landed as
https://crrev.com/86984d5fc56ce00e7b37ebe798aefa1d5374c83e
Cr-Commit-Position: refs/heads/master@{#339971}