Add error message when isolate.py trace has a return code other than 0.
Add error message when isolate.py trace has a return code other than 0.

R=maruel@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/11295002

git-svn-id: d5a9b8648c52d490875de6588c8ee7ca688f9ed1@164063 0039d316-1c4b-4281-b951-d872f2087c98

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