Add better_errors gem

* Add better_errors gem for nicer exception pages with live REPL
* https://github.com/charliesome/better_errors

Minor:

* Sort gem groups in a more logical fashion with main first
* Add comments to prevent `better_error` use in production
* Add comment to mocha gem explaining why it's listed last

Trello: https://trello.com/card/add-better-errors-gem/50b3ffc4c7cd26072c00ebfd/155