Throw custom pyramid swagger exceptions instead of pyramid httpexception...

We want to throw custom pyramid swagger exceptions for request and response validation so that we can catch them inside our tween. Within our tween, we didn't want to catch for HTTPInternalServerError or HTTPClientServerError as they're too broad (i.e, these can also be raised from pyramid, and not just pyramid_swagger). 
Please drop `PyramidSwagger` from the name. It should be obvious from an import line where the request validation error is coming from, and can always be changed with `from .. import .. as`. Good idea about subclassing `HTTPClientError` for backwards compatibility.