Handle missing reference files instead of crashing when copying them to the results dir.

This was discovered when trying to import various w3c tests, which appear
to have non-utf8 reference files and/or missing references in some places.
We should get those fixed, but we should also not crash the harness.

R=ojan@chromium.org, abarth@chromium.org
BUG=329113

Review URL: https://codereview.chromium.org/93883005

git-svn-id: svn://svn.chromium.org/blink/trunk@164044 bbb929c8-8fbe-4397-9dbb-9b2b20218538