Prior to this commit, all forward outcomes resulted in a 404. This
commit changes request and data guards so that they are able to provide
a `Status` on `Forward` outcomes. The router uses this status, if the
final outcome is to forward, to identify the catcher to invoke.
The net effect is that guards can now customize the status code of a
forward and thus the error catcher invoked if the final outcome of a
request is to forward.
Resolves#1560.
