Better handling of exceptions when walking parse trees

[degory]

Handling exceptions when walking parse trees is messy and doesn't always leave the diagnostics stack and other state consistent. This is mainly caused by having separate pre() and visit() methods in the visitors. Ideally we'd have a single method to override in the visitor per parse tree, so we can store any state in local variables, and that state can be restored from a single try/catch/finally wrapping all of the pre-walk actions, the walk over all children, and the post-walk (visit) actions.