only raise subclasses of Response in our algorithm funcs

[chadwhitacre]

Reticketed from #249. The purpose is to provide fine-grained control over handling specific exception cases.

[chadwhitacre]

IRC

The idea is that we'd do raise FileNotFound() during dispatch instead of raise Response(404). Each exception subclass would be raised from exactly one place, so you'd have fine-grained control when catching. And if we implement this right, then @zwn could monkey-patch FileNotFound to provide custom behavior without having to implement an algorithm function to catch it.