Fix PhutilUTF8TestCase::testUTF8Convert for PHP 8

Summary:
In PHP 8 passing an invalid encoding to mb_convert_encoding raises a
ValueError (which extends Error not Exception), so fix the test to also
catch Throwable (but leave the explicit Exception case for PHP 5, which
lacks Throwable).

Test Plan: Ran arc unit --everything

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21501