Allow to register an onerror callback

I'm using HolderJS in an emberjs-app and add and remove a lot of pictures during a single app run.

As Holder doesn't provide a way to deregister an image today, I see a lot of `Holder: placeholder is not visible` errors from already removed images.

This PR adds the possibility to register an onerror callback for Holder to handle those errors in a custom way.

Example:
```js
Holder.onerror = function(error) {
  // Log, but ignore errors
  console.log('Holder.js', error);
}
Holder.run();
```