Add exception handler for unhandled exceptions

When exceptions are thrown and not caught, these will result in:

• display_Errors = Off: 500 error page (so called white page)
• display_Errors = On: Fatal error displayed by PHP default exception handler
• Environment "development": Symfony exception handler displays nice to view exceptions page (not specific to phpBB)

In order to improve this, we should implement our own exception handler that displays a better looking exception display similar to Symfony's own exceptions page. This should be adapted for phpBB and only display exception details when it should (similar to error handler), e.g. for admins or when debug is enabled on purpose.