Don't fail silently?

[james-d-mitchell]

At present there are a number of operations (such as GraphvizRemoveNode) that do nothing if their arguments are valid.
For example, doing GraphvizRemoveNode(gv, "non-existent-node"); just returns gv unaltered. This might be ok, but we should try to be consistent everywhere and do one of the following:

1. Nothing (currently what happens)
2. Issue a warning that "non-existent-node" doesn't exist and that the first argument will be unaltered;
3. Issue an error

There might be some merit in having code that just doesn't error out, but just carries on. On the other hand, I think this will violate the principal of least astonishment, i.e. I have a graph, I remove a node (mistyping its name), and no nodes are removed (surprised face!). I think issuing a warning is the minimum we should do, but think errors would be better (there's no way to inadvertently ignore an error whereas a warning can be disabled, swallowed by something else, ignored or just overlooked).

What say @mpan322 and @mtorpey ? I'm raising this issue for discussion about what's desirable, rather than as a TODO for anyone.

[james-d-mitchell]

Fixed by #28