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 !
I'd keep this (for werkzeug compatibility).