Fix error_handler type hints

[ScullWM]

Hi,

I've updated the error_handler typehinting, mostly the return type declarations.
From php.net documentation, the error_handler should return a bool and not a void (https://www.php.net/manual/en/function.set-error-handler.php).

Thanks to psalm 👍

ERROR: InvalidArgument - src/Doctrine/Instantiator/Instantiator.php:135:27 - Argument 1 of set_error_handler expects null|callable(int, string, string=, int=, array<array-key, mixed>=):bool, Closure(int, string, string, int):void provided
        set_error_handler(static function (int $code, string $message, string $file, int $line) use ($reflectionClass, & $error) : void {
            $error = UnexpectedValueException::fromUncleanUnSerialization(
                $reflectionClass,
                $message,
                $code,
                $file,
                $line
            );
        });

[lcobucci]

@ScullWM awesome, thanks!