Corrupted GIF fallback

@drsnyder @garthwebb 
When a thumbnail request for an animated gif fails, fallback to thumbnailing the first frame of the gif. This assumes the first frame isn't corrupted, which is probably OK.

example broken image: http://vignette.wikia.nocookie.net/glee/e/e6/1goyourownway.gif/revision/latest/zoom-crop-down/width/152/height/104
original: http://vignette.wikia.nocookie.net/glee/e/e6/1goyourownway.gif/revision/latest
thumb'd first frame: http://vignette.wikia-dev.com/glee/e/e6/1goyourownway.gif/revision/latest/zoom-crop-down/width/152/height/104