So you're with fine the @concrete-extensible stuff?

Seems fine to me, generally.

In the next iteration of Conduit, I'd like to push all the wacky error handling more heavily into exceptions, too. For example, we have like 20 methods that define some custom version of "you're trying to update an object which does not exist", and all of that should just be returning something like a 404 response (and maybe even just a 404 response literally).

Yeah. We can rebalance the class tree later (e.g., have an abstract base and then a bunch of final subclasses) if necessary, but it's reasonable to put that off until the next significant iteration since it will probably muck with things anyway.

(And I don't think there's anything inherently bad about a tree like this.)