Check for running light-locker instance, exit with failure if not found

The current version of light-locker and light-locker-command do not check for a running instance of light-locker.

So for this scenario, the session is unprotected:
killall light-locker
light-locker-command -l

This takes you to the login screen with a false sense of security.  The changes below add a check for running light-locker process, and exits with an error if one is not found.  

Please consider this item for a point release, 1.2.1, as it is a significant security concern.