Make error messages about private scripts more descriptive

Instead of just crashing with RELEASE_ASSERT(), we should print more descriptive
error messages to help debug private scripts.

BUG=341031

Review URL: https://codereview.chromium.org/389913003

git-svn-id: svn://svn.chromium.org/blink/trunk@178046 bbb929c8-8fbe-4397-9dbb-9b2b20218538