fixed small bug in CallbackError constructor
fixed small bug in CallbackError constructor

It tried to get __name__ attribute from string returned by
pmcmds.getCmdName() insted of using it as is.

david-lassonde-adsk/pymel,shrtcww/pymel,juggernate/pymel,AtonLerin/pymel,shrtcww/pymel,assumptionsoup/pymel,leonsooi/pymel,juggernate/pymel,AtonLerin/pymel,assumptionsoup/pymel,leonsooi/pymel,david-lassonde-adsk/pymel