enable_motion_detection() when foscam is off causes error. Should exit gracefully with error code

[viswa-swami]

When foscam camera is off, and if we call 'enable_motion_detection()' then code returns an exception. code should gracefully handle it and return error code.

Below is error seen:

Send Foscam command: http://192.168.86.134:88/cgi-bin/CGIProxy.fcgi?usr=<username>&pwd=<password>&cmd=getMotionDetectConfig
Foscam exception: 
Traceback (most recent call last):
  File "./test.py", line 4, in <module>
    result = mycam.enable_motion_detection()
  File "/usr/local/lib/python3.5/dist-packages/foscam/foscam.py", line 626, in enable_motion_detection
    result = self.set_motion_detection(1)
  File "/usr/local/lib/python3.5/dist-packages/foscam/foscam.py", line 618, in set_motion_detection
    current_config['isEnable'] = enabled
TypeError: 'NoneType' object does not support item assignment

PR #16 fixes this issue.