Make the UI when you use an invalid cursor ("?after=19874189471232892") a little nicer

Summary:
Ref T13259. Currently, visiting a page that executes a query with an invalid cursor raises a bare exception that escapes to top level.

Catch this a little sooner and tailor the page a bit.

Test Plan: Visited /maniphest/?after=335234234223, saw a nicer exception page.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13259

Differential Revision: https://secure.phabricator.com/D20295