Improve error handling

[nextpulse]

Currently, the only way to check for error is to look at the content of the returned string. Not always easy, assume you need to know the actual content of the error.

kraken_ruby/lib/kraken_ruby/client.rb: r['error'].empty? ? r['result'] : r['error']

Maybe just throw an exception or return the complete json and let the app look for error field?