FIX: bare functions in callback registry on bogus unregister
FIX: bare functions in callback registry on bogus unregister

7de483b5fdaa99fa289b418ce0efc5102287ba64 added checking for the case
where the callback was wrapped by backcall, however if there are
un-wrapped functions registered and a non-registered function is
attempted to be unregistered then this fails by raise an uncaught
AttributeError.

ipython/ipython,ipython/ipython