Fix run-bindings-test crash on missing reference files

Currently if you add a new test and run r-b-t, it crashes,
as there's no reference file.
This fixes the crash and gives a diagnostic.

R=haraken

Review URL: https://codereview.chromium.org/229413005

git-svn-id: svn://svn.chromium.org/blink/trunk@171132 bbb929c8-8fbe-4397-9dbb-9b2b20218538