Drop next-route fallthrough from universal handler (#586)
Drop next-route fallthrough from universal handler (#586)

This has recently started causing `HEAD /` requests to fail on staging.
I'm not clear on why this has only started happening recently, but I
think this code has been wrong for some time: `handleMatch` is always
called and emits a response in all cases, so there's no reason to fall
through to other routes here (except for error-handling middleware,
which will still be called on unhandled exceptions).

Fixes #583.
canonical-websites/build.snapcraft.io,canonical-ols/build.snapcraft.io,canonical-websites/build.snapcraft.io,canonical-ols/build.snapcraft.io,canonical-websites/build.snapcraft.io,canonical-ols/build.snapcraft.io