Fix typo getting error stack in worker.
Fix typo getting error stack in worker.

zqzhang/testharness.js,RatneshThakur/testharness.js,bboyle/testharness.js,hallvors/testharness.js,jyasskin/testharness.js,ayg/testharness.js,jyasskin/testharness.js,hallvors/testharness.js