More explicit error when walking a tree and a node type isn't registered

[neoncube2]

Fixes #829

[marijnh]

Doing the check for when baseVisitor is undefined on every single node, instead of once at the start of a walk, doesn't necessarily seem like an improvement. Could you revert that part? Also, I think the error message would be clearer if it said `No walker function defined for node type ${type}`.

[neoncube2]

Oh, whoops, I didn't realize that check was in a loop XD

Fixed, and the error message has also been changed :)

[marijnh]

Thank you!

[neoncube2]

No problem, and thanks for the review! :)