Policy test server can simulate errors in more requests

PolicyTestServer can now simulate errors for device_attribute_update
and policy fetch requests.

Bug: 950471
Change-Id: I3b9bf3c84dbe500dec34254b8e8ebf06225c359e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1556811
Commit-Queue: Denis Kuznetsov <antrim@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649083}
6 files changed