Clean up ErrorHandler

[Funfare]

cleans up error handler as requested in #3503

TestCase was extended to controll behavior of error handler

$this->convertErrorsToExceptions(false);
$this->convertDeprecationsToExceptions(false);
$this->convertNoticesToExceptions(false);
$this->convertWarningsToExceptions(false);

[oliverklee]

@FunfareNico To fix the build, you can run this and then re-push:

./tools/php-cs-fixer fix

[sebastianbergmann]

Thank you for your contribution. I appreciate the time you invested in preparing this pull request. However, I have decided not to merge it.