WIP
Add ability to override call/2 for per controller error handling
Resolve conflicts
Enhance error handling with option for custom handler

Allow custom `page_controller` to be configured on Router , where `not_found/2`
and `error/2` actions can defined for custom 404 and 500 pages/handling.

Add `catch_errors` Router config to optionally disable errors being caught.

Add `debug_errors` Router config to optionally show Phoenix's 404/500 debug pages
Improve moduledocs
Update template config for new options
Add default 400/500 pages to template
Document error options and handling
Better explain error handling
Add example of catching errors at Controller layer
Bump outdated docs
Rename page_controller to error_controller
Resolve conflicts