Changed 'lockedBy' function internal code

It will continue to work the same for any code that is already
using it. It now returns a third value in its tuple: an exception
object (if any exceptions occured) or 'None'. Also, the way that
real names are discovered no longer directly depends on LDAP,
making the code slightly more portable.
Merge pull request #130 from eestrada/master

Changed internal code of 'lockedBy' function