Check for sniffer promiscuous permissions

[mjoblin]

Attempting to run nd-snifty without permissions to enter promiscuous mode generates a stack trace. It would be nice to detect this and produce a more useful message to the user.

Here's the stack track:

Traceback (most recent call last):
  File "/usr/local/opt/python-3.5.2/lib/python3.5/multiprocessing/process.py", line 249, in _bootstrap
    self.run()
  File "/usr/local/opt/python-3.5.2/lib/python3.5/multiprocessing/process.py", line 93, in run
    self._target(*self._args, **self._kwargs)
  File "/home/pi/venv/nd/lib/python3.5/site-packages/netdumplings/console/snifty.py", line 101, in network_sniffer
    sniffer_kitchen.run()
  File "/home/pi/venv/nd/lib/python3.5/site-packages/netdumplings/dumplingkitchen.py", line 175, in run
    sniff(filter=self.filter, prn=self._process_packet, store=0)
  File "/home/pi/venv/nd/lib/python3.5/site-packages/scapy/sendrecv.py", line 572, in sniff
    s = L2socket(type=ETH_P_ALL, *arg, **karg)
  File "/home/pi/venv/nd/lib/python3.5/site-packages/scapy/arch/linux.py", line 468, in __init__
    self.ins = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.htons(type))
  File "/usr/local/opt/python-3.5.2/lib/python3.5/socket.py", line 134, in __init__
    _socket.socket.__init__(self, family, type, proto, fileno)
PermissionError: [Errno 1] Operation not permitted