Ensure that we don't get stuck with a bad message in queue

If a bad JSON message is received, the begin/rescue will be executed
without acknowledging handling of this (bad) result. So we end up with no more messages being processed because of a bad result that MultiJson can't parse (UTF-8 issue like in #783).

I've move it out of the begin/rescue block to make sure we do ACK the message (and have it "disappear").

It works so far in our dev environment, thought I'm not 100% sure if this is the best/correct way to handle this case.