Fix let pattern error accumulation

Fix let pattern error accumulation

When the checker was added prohibiting lexical binding called let,
certain error propagation was not implemented properly. This patch
fixes that issue, which fixes error checking for cases such as
  let [let]

BUG=v8:4403
R=adamk
LOG=N

Committed: https://crrev.com/b802051df26200c346db16e8e0b85358b57bb0e9
Cr-Commit-Position: refs/heads/master@{#31289}

[Dan Ehrenberg, 2015-10-15 09:51:31]

The CQ bit was checked by littledan@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-15 09:51:38]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1409613003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1409613003/1

[adamk, 2015-10-15 09:53:53]

lgtm

[commit-bot: I haz the power, 2015-10-15 10:41:10]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-15 10:41:11]

Dry run: This issue passed the CQ dry run.

[Dan Ehrenberg, 2015-10-15 10:44:51]

The CQ bit was checked by littledan@chromium.org

[commit-bot: I haz the power, 2015-10-15 10:45:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1409613003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1409613003/1

[commit-bot: I haz the power, 2015-10-15 10:45:59]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-15 10:46:22]

Patchset 1 (id:??) landed as
https://crrev.com/b802051df26200c346db16e8e0b85358b57bb0e9
Cr-Commit-Position: refs/heads/master@{#31289}