Use Tracy for error handling

[jtojnar]

Tracy produces much nicer error messages and allows us to get rid of F3.

The version supporting PHP 5.6 unfortunately produces warnings on PHP 8.0. Also until Tracy 2.9 (for PHP 7.3+) starts a session, which conflicts with session_set_cookie_params.

• [ ] we need to convince tracy to log even in dev mode – likely by subclassing
• [x] ~~Find out why ajax requests do not appear in tracy panel~~ seems to work with 2.9
• [ ] integrate PDO into tracy panel