zdtm: suppress useless error messages

Start test
./mxcsr --pidfile=mxcsr.pid --outfile=mxcsr.out
Run criu dump
Unable to kill 44: [Errno 3] No such process <--------------- this one
Run criu restore
Run criu dump
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com>

test/zdtm.py

@@ -313,8 +313,7 @@ def wait_pid_die(pid, who, tmo = 30):
 	while stime < tmo:
 		try:
 			os.kill(int(pid), 0)
-		except Exception, e:
-			print "Unable to kill %d: %s" % (pid, e)
+		except OSError, e:
 			break
 
 		print "Wait for %s(%d) to die for %f" % (who, pid, stime)