ait-run: Stop if the client fails

The client must return non-zero if it can't run, emitting a message
to clarify why it couldn't.

ait-run will detect this and stop.

Implement this on the netperf-udp_stream test, detecting when the
netserver daemon is not running, i.e. if it can't establish the
connection.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>