lua: Check for bad invalid datasets before binding them.
lua: Check for bad invalid datasets before binding them.

If the dataset fails to load, we don't want to bind any methods to
it.  Better to just not initialize our internal Lua pointer for
it, then.

git-svn-id: 83c4c97093aeea62ac982fbfd516bf9db27f5c99@2526 c36c8488-0289-0348-9b64-b301f74bd9a7

BlueBrain/Tuvok,BlueBrain/Tuvok,BlueBrain/Tuvok