Re: [w3c/IndexedDB] Integrate with window.onerror (#49)

The propagation from workers to window does not appear to behave as described in https://github.com/w3c/IndexedDB/issues/49#issuecomment-237172028 in Firefox 57 for Indexed DB errors.

Given a page with a worker that uses IDB that has handlers for the request, transaction, connection, a global onerror handler in the worker, an onerror handler on the Worker object, and an onerror handler on the window global scope, Firefox sees the following when a ConstraintError is triggered:
```
in worker: [object IDBRequest]: error, ConstraintError
in worker: [object IDBTransaction]: error, ConstraintError
in worker: [object IDBDatabase]: error, ConstraintError
worker global onerror: ConstraintError, http://localhost:8888/worker.js, 22, 17, null
```
(i.e. the propagation path is just IDBRequest → IDBTransaction → IDBDatabase ⇒ Worker global)

In contrast, for a non-IDB error, e.g. the script ends with `throw Error('oops')`:
```
worker global onerror: Error: oops, http://localhost:8888/worker.js, 33, 0, Error: oops
Worker object onerror: [object ErrorEvent], undefined, undefined, undefined, undefined
window global onerror: Error: oops, http://localhost:8888/worker.js, 33, 0, null
```
(i.e. the propagation path is Worker global ⇒ Worker object ⇒ Window global)

Further, on the worker's global onerror, the first argument passed is the **Error** not the **Event**. That means `preventDefault()` can't be called.

@bevis-tseng - can you confirm the above, and if this is intentional?

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