Test that the error event can be canceled during loadstart

The first of these tests fails in Blink.

The spec probably doesn't require the tested beahvior, but it should:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=24710
Merge pull request #685 from foolip/load-removes-queued-error-event

Test that the error event can be canceled during loadstart