Returning false in #save on a server error.

[swilliams]

Right now Arkenstone::Document returns self at the end of #save. If the server returns a 500, there isn't an easy way to discover this. I hacked in a server_errors property real quick to get something done, but this should be addressed better.

I did create a branch to demonstrate this, and all of the tests pass, but I don't know if there is other code out there depending on the old behavior.

Thoughts before I make a PR?