Handle case of __name__less callables

[MikeHart85]

Minor edge case. Some callables do not have a __name__ attribute.

For example:

class Foo(object):
    def __call__(self, x):
        print(x)

>>> foo = Foo()
>>> foo('bar')
bar
>>> foo.__name__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Foo' object has no attribute '__name__'

This applies to partials as well.

This PR will display repr(func) when func.__name__ is not available.