Expose the update engine function GetLastAttemptError to dbus clients

This is in preparation for getting the last attempt error code on the chrome
side in order to show the user appropriate actionable error messages.

BUG=chromium:581071
TEST=emerge-peach_pit system_api

Change-Id: I5af9de0f55331f273408cdcfc8daec9c35612f46
Reviewed-on: https://chromium-review.googlesource.com/328849
Commit-Ready: Ahmed Fakhry <afakhry@chromium.org>
Tested-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
1 file changed