Add onError callback instead of throwing error

[camdarley]

Hi,
The React img component has an onError callback in case there's something wrong with image src.
Would it be possible to provide the same callback in the Spritesheet component?
Currently, Spritesheet throw an error:

 imgLoadSprite.onerror = () => {
      throw new Error(`Failed to load image ${imgLoadSprite.src}`);
 };

could be replaced by:

 imgLoadSprite.onerror = () => {
      onError(`Failed to load image ${imgLoadSprite.src}`);
 };

Regards