env["action_dispatch.exception"] not being set in ActionDispatch::DebugExceptions

Hi there!

I'm having hard time figuring out why `ActionDispatch::DebugExceptions` is not setting this when exception occurs. Was this intentionally left out? 

With this, `env["action_dispatch.exception"]` stores the exception, regardless of the `env["action_dispatch.show_detailed_exceptions"]`.
 
Oh yeah, no tests for now, since I wanted to get some feedback first. :smile_cat: