Instrument pwrite() failure.
Instrument pwrite() failure.

This will shed more light on the circumstances in which
DeltaPerformerTest.RunAsRootSmallImageTest is failing.

BUG=chromium-os:31077
TEST=Builds and passes unit tests

Change-Id: I9b97191b62ef6cf3b39bdb68fa40d758b0632283
Reviewed-on: https://gerrit.chromium.org/gerrit/27185
Reviewed-by: Don Garrett <546ec366615570e68497e3c5df9092106a9808f4@chromium.org>
Commit-Ready: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>
Tested-by: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>

marineam/update_engine,endocode/update_engine,marineam/update_engine,endocode/update_engine,marineam/update_engine,mjg59/update_engine,marineam/update_engine,endocode/update_engine,mjg59/update_engine,mjg59/update_engine