Add Error callback for when images requests fail

[travisdahl]

It would be nice to have a "failed" event too if image request failed. For instance in my app I am adding a ".loaded" class to images that have finished using the 'load' event and then for various reasons i am periodically checking to see if all images were loaded, but if an image failed '404' then my check runs infinitely. If there was a 'failed' event I could add a failed class or even a 'loaded' class, but something to know that the plugin itself ran, but the image request failed. Does that make sense?

[tusharmath]

@tuupola Please merge this. This is a very relevant pull request.

[dmccabe]

+1. This would be very useful to have.