Write failed snapshot (#70)

[ninogresenz]

Hey, first of all, I started to learn go couple month ago, and I used this lib already in a couple projects. I really like to use it but I also found it a bit hard to read bigger diffs sometimes. So I gave it a shot to implement this. Please let me know if something is wrong or could be done better. I'm looking forward to your feedback.

P.S.: this is my first open-source contribution ever, so please be nice. ;)