Have Sites pages trap and display system exceptions

[davidhabib]

Currently, if a Sites page hits any type of exception, the unauthenticated guest user will just see a generic salesforce error saying you don't have permissions to view the site (Authorization Required page).

Ideally, the Sites pages would trap these exceptions and display them on the page, to make it easier for admins to debug what might be going wrong.

hub thread: https://powerofus.force.com/0D58000003IEdAG

[davidhabib]

**lurch: add

[LurchTheButler]

Tracking W-018145

[coriobriensfdo]

Another Thread.