charmstore/router: generate http.StatusNotFound appropriately

This generates messages (and error codes) as expected by the API specification.
It's precisely that distinction we are currently making - between "not found" (you're trying to address something that's genuinely not there) and "metadata not found" (we know what you're trying to address, but it has no content). The former would probably generate an error in a client; the latter is expected and probably would not.

I will add comments.