dispatch error event only if iron-ajax-error allows it.

[StefanLiebenberg]

This change adds the ability for listeners of "iron-ajax-error" to prevent "error" event.

ajaxElement.addEventListener('iron-ajax-error', function (event) {
     event.preventDefault(); // now "error" will never fire.
})

[e111077]

Great feature. Can you write a simple test and also rebase onto master?