Add a message if check_nrpe can't connect

If check_nrpe can't connect it prints on stderr (which we ignore) and it
returns 255. It's the only case where it returns this code, according to
check_nrpe.c.

mownitoring.py

@@ -133,6 +133,8 @@ def check_status(check, host, port, machine, notifiers, conn):
     """Choose if we send an alert."""
     timestamp = datetime.datetime.now()
     status, message = check_nrpe(check, host, port)
+    if status == 255:
+        message = "Connection refused"
     c = conn.cursor()
     param = (machine, check)
     try: