Currently, worker exception handling doesn't match the spec (distilling section 4.7 of the Web Workers spec and section 220.127.116.11 of the HTML5 spec):
1) We need to expose errors via WorkerContext.onerror
2) If WorkerContext.onerror() does not return false (HTML5 18.104.22.168), then we need to forward the error to the Worker object.
3) If the event is not cancelled by the error handler, then it is reported to the user via the error console (or forwarded up to the parent Worker object, in the case of nested workers).
One other thing to note - the spec currently claims that error events in workers are dispatched via the queue. This language in the spec is going away - they will be dispatched synchronously, just like error events in pages.
*** This bug has been marked as a duplicate of 25150 ***