Add whoops for in_dev error handling

[Lavo]

When we devs have in_dev enabled (or whatever the equivalent may be in 4.x) could you integrate this error handler into the core?

http://filp.github.io/whoops/

Sure, you could say it's up to the dev to install that but that could require us to potentially reimplement it / swap our your own implementation every time an upgrade is performed.