render rack exceptions using application controller in rails 3

test plan:
* (note: if testing in development mode, set
 `config.consider_all_requests_local = false`
 in config/environments/development-local.rb)

* try to visit a page that does not exist (e.g. '/ponies')
* the error page should be formatted the same way as in
rails 2

fixes #CNVS-12178

Change-Id: I80c5d96f23bc8fd39b27edd7a845beeea0b2f300
Reviewed-on: https://gerrit.instructure.com/32958
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>