Add a link when the isolated file is missing.
Add a link when the isolated file is missing.

This points new isolate users to a quick FAQ entry which tell them how to get the required .isolated files.

R=maruel@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/11419162

git-svn-id: d5a9b8648c52d490875de6588c8ee7ca688f9ed1@169490 0039d316-1c4b-4281-b951-d872f2087c98

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