I'm looking at docs/architecture.rst and at/around lines 463-473, I'm pretty sure this is not at all accurate - the Akhet I have installed here just doesn't behave this way. I'm raising exceptions of all kinds just fine, rather than returning them, and have no specific exception view to handle them.
In fact, I'm trying to define one to handle something specific and having little luck having it actually be caught, but I'm sure that's a local issue.
Lines 475-478 is also a little misleading as worded, since the NotFound and Forbidden exceptions are identical to HTTPNotFound and HTTPForbidden, respectively (ref: module pyramid.exceptions). And Pyramid doesn't appear to actually do anything different with these - the only one that seems specific is 500, when debugging is enabled.