Guard corrupted json

This PR adds a guard against the messages coming from the rabbit broker.  It is possible, and has been happening to me in production, that in between the client sending metric data and it getting to sensu-server it is getting corrupted.  This is a transmission error but since there's no way to fix the network or the message I'm choosing to log the error and let the message go.

Added the rescue to the client daemon as well for consistency.
The Sensu logger supports a hash for log event context, we should be using it instead of large strings.