Fix #688: Remove return value of JSEnv#runJS.

JSEnv#runJS used to return Some(<string>) if it failed due to
historical reasons. This is no longer necessary and introduces useless
clutter and confusion (e.g. runners failing without the outer code
failing). runJS now throws an exception if it fails.
Merge pull request #691 from gzm0/cleanup-runner

Fix #688: Remove return value of JSEnv#runJS.