Add a simple primitive for managing PHP runtime error logs

Summary:
Ref T13624. If we want to send PHP errors to a log, using the "error_log" configuration option catches the broadest set of errors across versions of PHP.

Configuring this disables errors on stderr, since they're sent to the log instead. We'd like them to go to both places; provide a simple wrapper for this. Also do a bit of writability testing.

Test Plan: Wrote errors to a new log, see followup changes.

Maniphest Tasks: T13624

Differential Revision: https://secure.phabricator.com/D21578