Use new Symfony Error Controller, instead of Twig's old one.

[bobdenotter]

Fixes #1476

Part of #1182

[peterboorsma]

I think this commit causes problems when updating to RC25.
Frontend gives a Internal Server Error. Log says (no error message in my browser screen):

{"exception":"[object] (ErrorException(code: 0): User Deprecated: The "Symfony\Component\Debug\FatalErrorHandler\ClassNotFoundFatalErrorHandler" class is deprecated since Symfony 4.4, use "Symfony\Component\ErrorHandler\FatalErrorHandler\ClassNotFoundFatalErrorHandler" instead. at /Users/xxxx/localdev/xxxx/vendor/symfony/debug/FatalErrorHandler/ClassNotFoundFatalErrorHandler.php:20)"} []

[bobdenotter]

The solution should be the top item in this file: https://github.com/bolt/project/blob/master/UPDATE.md

After making these changes, run bin/console cache:clear, and you should be up and running again.