Tunnel keep alive not working

[thilux]

The tunnel keep alive is not working. It fails with the following exception:

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 763, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/lib/python2.7/site-packages/sshmetroserver/main.py", line 188, in keep_live_metros_alive
    if not util.is_server_alive(metro.metro_host, metro.metro_port):
  File "/usr/lib/python2.7/site-packages/sshmetroserver/util.py", line 32, in is_server_alive
    except ConnectionRefusedError:
NameError: global name 'ConnectionRefusedError' is not defined