Improve window.onerror handling

Improve window.onerror handling

* Chain in on other window.onerror handlers and honor their
  return value. Just like we expect other code to keep our onerror
  handler (e.g TestSwarm).

* Added explicit 'return false' for clarity instead of leaving it
  undefined and assuming other handlers understand that.

* Best reviewed with -w :)

* grunt: Passes, without errors.