Summary: Ref T1806. Ref T7173. Depends on D14047. Currently, all exception handling is in this big messy clump in `AphrontDefaultApplicationConfiguration`. Split it out into modular classes. This will let a future change add new classes in the Phacility cluster which intercept particular exceptions we care about and replaces the default, generic responses with more useful, tailored responses. Test Plan: {F777391} - Hit a Conduit error (made a method throw). - Hit an Ajax error (made comment preview throw). - Hit a high security error (tried to edit TOTP). - Hit a rate limiting error (added a bunch of email addresses). - Hit a policy error (tried to look at something with no permission). - Hit an arbitrary exception (made a randomc ontroller throw). Reviewers: chad Reviewed By: chad Maniphest Tasks: T1806, T7173 Differential Revision: https://secure.phabricator.com/D14049