Duck typing is great let's do more of it

Don't hardcode which exception is to be considered an HTTP error, instead
embrace the duck typing philosphy and make any exception that have a get_response
method an HTTP Exception.

The rational behind this, beside the obvious advantage of duck typing over hardcoded
exception and subclass check, is that we don't want to be stuck with using Werkzeug
exception classes, since this later is hard to extend !
Imo, duck typing is mostly broken in dynamic languages. You usually need full interfaces to do something useful with an object, which is tedious to check manually. In some cases it's useful though (since the interface only has one method), and this might be one of them.