Add support to store logs of failed tests.
Add support to store logs of failed tests.

This is useful to track each error individually.

R=csharp@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/12681021

git-svn-id: d5a9b8648c52d490875de6588c8ee7ca688f9ed1@190962 0039d316-1c4b-4281-b951-d872f2087c98

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py