This adds a test case that ensures calling Debug.scripts without any
listener attached fails gracefully. For now we are throwing the string
"illegal access", this might change in the future to be a dedicated
exception.

R=yangguo@chromium.org
TEST=mjsunit/debug-scripts-throw

Review URL: https://codereview.chromium.org/1411193002

Cr-Commit-Position: refs/heads/master@{#31377}