Change error message raised when free variable is not yet bound. It

Change error message raised when free variable is not yet bound.  It
now raises NameError instead of UnboundLocalError, because the var in
question is definitely not local.  (This affects test_scope.py)

Also update the recent fix by Ping using get_func_name().  Replace
tests of get_func_name() return value with call to get_func_desc() to
match all the other uses.