Fixing the Jails stop mechanism

The Main issue here was the fact that warden's way of learning
whether the jail was running or not, was to check the jls
command OR check if stuff was mounted in the jail. So as a
compromise I just disabled the mounts check.

Other Fixes:
1. The common/system.py umount(path) function was returning False.
   This is now changed to raising an error.
2. The jails/models.py's mount and umount functions were returning
   self.mounted and not self.mounted respectively, which is a nice
   double safety-net, but since mounted maps to common/system.py,
   which again is only a True/False flag, this in-turn disables
   error reporting to the user.
3. There was a "FIX ME--better error handling, show the user why"
   statement (in RED!) in jails/form.py relating to the umount
   function. Doing 1 & 2 fix this, and the necesaary error raising
   is now in place.

Ticket: #6227