Fix handling of failed distillation.

Fix handling of failed distillation.

Currently, when distillation fails, the TaskTracker signals that a
source has finished. It then checks if all sources are done (and if not,
it will wait to call the callback). The check for sources done though
assumes that if distiller_ is non-null, then distillation is still in
progress. So, we should be deleting/releasing distiller_ when it
finished (and before signalling that the source is finished).

This also updates the standalone content extractor to fail if extraction
of some url fails.

Committed: https://crrev.com/39186c37d96231e4b48fcc63bf65d4100b0b6735
Cr-Commit-Position: refs/heads/master@{#295599}

[cjhopman, 2014-09-16 20:30:20]

cjhopman@chromium.org changed reviewers:
+ nyquist@chromium.org

[cjhopman, 2014-09-16 20:30:21]

nyquist: *

[nyquist, 2014-09-18 21:49:01]

lgtm

[cjhopman, 2014-09-18 21:58:21]

The CQ bit was checked by cjhopman@chromium.org

[commit-bot: I haz the power, 2014-09-18 22:00:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/575883002/1

[commit-bot: I haz the power, 2014-09-18 23:33:32]

Committed patchset #1 (id:1) as bcb95a9710651a5feb3123fca6746f6793946944

[commit-bot: I haz the power, 2014-09-18 23:34:31]

Patchset 1 (id:??) landed as
https://crrev.com/39186c37d96231e4b48fcc63bf65d4100b0b6735
Cr-Commit-Position: refs/heads/master@{#295599}