Add onError to ReactDOMImg to complement onLoad

@spicyj added the the useful `onLoad`, lets complement it with `onError` for those times the images doesn't load. :)

It is *possible* that we may want to overload `onLoad` to sometimes emit `onError` instead. I've seen people comment in various places that sometimes images fails to load but emit `onLoad` with 0 width/height image instead. But I wasn't able to reproduce it myself in IE8-11 and edge versions of browsers.