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.

• 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).