don't drop the original message on parsing stats

[wdpypere]

the current setup drops the nagios message if there was one, and replaces it by the statistic name. so it might be

msg = "failed in service nrpe" 
stats= {'fail': 1}

and it will return just fail as a message.
The only place where I see this function being used correctly is in service_sanity. other uses just dance around it and do checking themselves. (which is wrong, but another discussion)

In fact, it does this for OK values, but not for failed values, which is counterintuitive.