Gracefully handle i2c (SMBus) library being installed but i2c not enabled

[WayneKeenan]

Traceback (most recent call last):
File "/usr/lib/python3.4/runpy.py", line 170, in _run_module_as_main
"main", mod_spec)
File "/usr/lib/python3.4/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "/usr/local/lib/python3.4/dist-packages/picraftzero/main.py", line 26, in
wheelbase = Wheelbase(left=0, right=1)
File "/usr/local/lib/python3.4/dist-packages/gpiozero/devices.py", line 135, in call
self = super(GPIOMeta, cls).call(*args, **kwargs)
File "/usr/local/lib/python3.4/dist-packages/picraftzero/zero.py", line 211, in init
left_motor=PiCraftMotor(left),
File "/usr/local/lib/python3.4/dist-packages/gpiozero/devices.py", line 135, in call
self = super(GPIOMeta, cls).call(*args, **kwargs)
File "/usr/local/lib/python3.4/dist-packages/picraftzero/zero.py", line 161, in init
Motor = get_motor_provider()
File "/usr/local/lib/python3.4/dist-packages/picraftzero/providers/init.py", line 44, in get_motor_provider
find_devices_and_import()
File "/usr/local/lib/python3.4/dist-packages/picraftzero/providers/init.py", line 16, in find_devices_and_import
devices = i2c_scan() # TODO: add config option to specify i2c bus
File "/usr/local/lib/python3.4/dist-packages/picraftzero/utils.py", line 197, in i2c_scan
bus = smbus.SMBus(bus_num) # 1 indicates /dev/i2c-1
FileNotFoundError: [Errno 2] No such file or directory