Error propagation for cmd.get_object_list()

[speleo3]

Propagates cmd.get_object_list() errors from C++ to Python.

Does not change the empty string behavior yet
Changes the empty string behavior (#478)

Alternative to #481

Note: Suggest not to squash, second commit is deliberate to isolate the empty string behavior change

[pslacerda1]

I think there is no reason to return None instead of raising an exception. Or return an object list of zero or more elements, or throw errors.

[speleo3]

I think there is no reason to return None instead of raising an exception

I do agree. I just wanted to keep that a separate change. I added a second commit now.

[pslacerda1]

Perfect, thank you.

[JarrettSJohnson]

Thanks to the both of you for the discussion and contributions.