Fix error handler on PHP 8

Summary:
PHP 7.2.0 deprecated the 5th parameter and PHP 8 removed it, so stop
using it and provide a default value to avoid erroring with:

Too few arguments to function PhutilErrorHandler::handleError(), 4 passed and exactly 5 expected

Test Plan: Used to create this revision with PHP 8 on macOS

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21498