Fail quietly when failing to write access log

Summary: See D5874. PhutilDeferredLog's exception on write()/__destruct() is not especially important and can come at an awkward time. Instead of throwing, just emit an error message to the log.

Test Plan: Faked failed writes and saw an error message in the log instead of many terrible things everywhere.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3144

Differential Revision: https://secure.phabricator.com/D5875