This change adapt the auth combinator example to the new router code.
In general, the server interpretation of user-written combinators will
be affected by the new routing code.
The change here also introduces a change in functionality: previously,
wrong authentication triggered a "hard failure", whereas we now trigger
a "soft failure", which is recoverable. For the simple example, this
does not make a lot of difference.
In general, I think having a soft failure is the right option to take
here, although we want a more general story about the relative
priorities of different error codes.
