Allow using custom error handler

[jtojnar]

I would like to use Tracy for handling errors and exceptions since it has interesting features like AJAX error handling.

[Rayne]

Fat-Free Framework doesn't forbid to use a third party debugger or error handler. I did a few successful tests with a custom ONERROR handler which forwards the errors to Tracy.

Two notes:

• I didn't test the AJAX part
• The snippet is using the public method Debugger::exceptionHandler which is declared as @internal.

$f3->set('ONERROR', function () use ($f3) {
    Debugger::barDump($f3->get('ERROR'), 'ERROR');
    $e = $f3->get('EXCEPTION');
    // There isn't an exception when calling `Base->error()`.
    if (!$e instanceof Throwable) {
        $e = new Exception('HTTP ' . $f3->get('ERROR.code'));
    }
    Debugger::exceptionHandler($e);
});

Full example: Fat-Free Framework with PHP Debugger Tracy

[jtojnar]

The issue with using ONERROR is precisely that it requires us to connect fatfree with whatever handler we want manually, rather than just using the other library’s defaults.

Additionally, even if we set ONERROR handler, fatfree still changes the HTTP status code, spams the stderr with error_log function and then dies, even though we might want to avoid any of that.