error handler does not run in jester 0.4.0
Am using jester 0.40 with nim devel channel on Ubuntu and have the contrived web app example below.
If I enter an unknown url (http://localhost:5000/meh) the 404 handler below doesn't run instead the handler from jester/private/errorpages.nim is the one that gets displayed
```
import jester

routes:
  get "/":
    resp h1("Hello world")
  error Http404:
    resp Http404, "Looks you took a wrong turn somewhere."

  error Exception:
    resp Http500, "Something bad happened: " & exception.msg'
```