Handle errors in reading cache

Summary: I've stored PhutilSafeHTML instance to cache on devbox and then wasn't able to read it in production.

Test Plan: Displayed revision with unreadable cache, saw error in error log but not fatal.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4520