Exception if invalid interface name is given on CLI

[lowell80]

If an invalid interface name is provided (like --interface 0), than an ugly exception message is provided instead of a human readable error. The user should be given better feedback.

Version:

python kintyre_speedtest.py --version
0.3.9

Example output:

# python kintyre_speedtest.py --interface 0

interface:  ['0']
mode=random    interface:  ['0']
DEBUG:  Payload --> https://X.XX.XX.XX:8443/services/collector/event  :  {"action": "ping", "hostname": "BLAH", "agent": {"name": "Lowell's Laptop", "org": "kintyre.co"}}
   Status code = 200
DEBUG:  iterfaces for testing: InterfaceInfo(ip=None, dev=None, meta={'_error': 'No non-blacklisted interfaces found.'})
DEBUG:  get_macosx_hardware() returns: {0: {'hardware_port': 'LPSS Serial Adapter (1)'}, 1: {'hardware_port': 'LPSS Serial Adapter (2)'}, 'en9': {'device': 'en9', 'hardware_port': 'AX88179 USB 3.0 to Gigabit Ethernet', 'ethernet_address': 'XXXXX'}, 'en0': {'device': 'en0', 'hardware_port': 'Wi-Fi', 'ethernet_address': XXXXX'}, 'en6': {'device': 'en6', 'hardware_port': 'Bluetooth PAN', 'ethernet_address': '8c:85:XXXXX'}, 'en1': {'device': 'en1', 'hardware_port': '..... REMOVE INFO .... }
Missing netsh
DEBUG:  get_windows_netsh() returns: None
Missing iwconfig
DEBUG:  get_linux_iwconfig() returns: None
Missing lshw
DEBUG:  get_linux_lshw() returns: None
Traceback (most recent call last):
  File "kintyre_speedtest.py", line 843, in <module>
    cli()
  File "kintyre_speedtest.py", line 839, in cli
    main(interfaces, out)
  File "kintyre_speedtest.py", line 381, in main
    info = if_.meta
AttributeError: 'NoneType' object has no attribute 'meta'

Also iterfaces should be interfaces.