Fix 500 in test results server when querying testlistjson

If the file doesn't exist in the repo, we should return a 400
instead of throwing an error and returning a 500.

TBR since this is causing 500s on the live server.

TBR=szager@chromium.org

Review URL: https://codereview.chromium.org/459243004

git-svn-id: svn://svn.chromium.org/blink/trunk@180016 bbb929c8-8fbe-4397-9dbb-9b2b20218538