FreeBSD: sensors_battery() raises OSError in case of no battery

[giampaolo]

This was signaled by @glebius and it happens on a FreeBSD system with no battery. Fix consists in making sensors_battery return None instead.

#PYTHONPATH=/usr/src/psutil python2 psutil/tests/test_connections.py

Traceback (most recent call last):
  File "psutil/tests/test_connections.py", line 30, in <module>
    from psutil.tests import AF_UNIX
  File "/usr/src/psutil/psutil/tests/__init__.py", line 156, in <module>
    HAS_BATTERY = HAS_SENSORS_BATTERY and psutil.sensors_battery()
  File "/usr/src/psutil/psutil/__init__.py", line 2316, in sensors_battery
    return _psplatform.sensors_battery()
  File "/usr/src/psutil/psutil/_psbsd.py", line 411, in sensors_battery
    percent, minsleft, power_plugged = cext.sensors_battery()
OSError: [Errno 2] No such file or directory