Fixes #56 - Returning an error to the client if the image was not succes...

Hi,

I am sending a PR for the #56 

I am checking for the presence of the word errorDetail in the response. Please let me know if there is a more efficient solution than this.
I don't quite get it - as long as there is an "errorDetail", we just assume it's Image not found?

What if the disk space on the docker machine is full? I'm assuming we'll be getting some I/O errors back, but we'll just translate it image not found.

Am I missing something?