Allowing hooks to return responses that indicate errors

Allowing hooks to return responses that indicate errors

Since response objects for failures (4xx/5xx responses) evaluate to False
in a boolean context, any hook which returns such a failure response will
evaluate to False.

The way hooks were setup, any failure response resulting from a hook would
be ignored, and the initial response before it got processed by the hook
would be substituted in its place. This commit changes that logic to test
for None so that hooks that return failures can do so.