(MCO-199) Fix halt_code to return the correct exitcode for mco ping

When calling halt_code with the client object created from a mco ping
application we are in the special case where we have 0 nodes discovered
and a non zero amount of responses.

This commit changes the list of "return if" statements in favor of a
if/else statement and adds the check where discovered = 0 but responses
> 0, allowing the method to behave as documented and ping to work as
> intended.
Merge pull request #205 from ploubser/mco-199

(MCO-199) Fix halt_code to return the correct exitcode for mco ping