Demo of custom error return value.

Result:

    Socket call
    Returned from: 20: 0x00000000fffffff3
    Traceback (most recent call last):
      File "cptbox_test_slave.py", line 2, in <module>
    Access: cptbox_test_slave.py
        socket.socket()
      File "/usr/lib/python2.7/socket.py", line 187, in __init__
    Access: /usr/lib/python2.7/socket.py
        _sock = _realsocket(family, type, proto)
    socket.error: [Errno 13] Permission denied