The browbeat stressng workload reports an error in the logs
while running the stressng command, even though the output of
the stressng command reports that it has executed successfully[1].
This happens in all iterations. Also, the iteration ends with END: OK
even if there happens to be a genuine error in executing the command.
This patch fixes these 2 issues by checking the exitcode of the stressng
command and raising an exception if it is nonzero.
[1] https://privatebin.corp.redhat.com/?dc09ee175a9ee377#37vYuNmR4CF2Hc3ghjcggXeW1EHY5WXDH5PB9Wb5sBoZ
Change-Id: I0d49ee6f80e4ae9b2946c67e8689daaccb200b1d
62650d6652