Move exceptions into a sub-namespace

[airbreather]

Something I neglected in 1.1.0 is to ensure that users have a decent experience when trying to discover Cursively's API through IntelliSense or the online API browser.

At the moment, all the new exception types added for the header-aware visitor simply live in the top-level "Cursively" namespace, which makes it harder to find the small handful of types that are meant to serve as starting points for the API.

Moving these to a Cursively.Exceptions namespace should help with this.

This would be a source and binary breaking change.