Add onError to ReactDOMImg to complement onLoad

[syranide]

@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.

[petehunt]

seems legit

[vjeux]

For FB people going to take this kind of code. When pulling in, make sure that you check for existing code that uses the same name using onError=. For onLoad it broke two call sites that accidentally transferred it to DOM nodes using this.transferPropsTo.

[syranide]

@petehunt Rebased.

[petehunt]

pulling in