Update GridLayer.js

'leaflet-tile-loaded' class was being applied even if a tile failed to load (404 error).  This would cause an image placeholder to appear on the map.

Here is an example:
![image](https://f.cloud.github.com/assets/2304500/1842716/6df771f4-74c1-11e3-830c-68a0e03f876c.png)

I fixed this by only adding the class if no error.