[5.5] Fix Whoops exception rendering

[thecrypticace]

The new Whoops setup is slightly broken. The defaults cause Whoops to output directly to the browser, call exit, and prevent testing when using the command line (because it returns an empty string when configured to not exit)

I've done a few things here:

1. A slight refactoring around Whoops and Symfony error handlers to simplify things.
2. Reconfigure whoops to always return a response to the exception handler
3. Use Symfony's error handler as a backup if configuring or running Whoops fails.
4. Fix a problem with rendering non-http exceptions that was hidden behind the fact that Whoops was calling exit.