Make sure the web log for daemons captures errors

Summary:
See T613. When you start daemons with "phd launch ...", they get run with
"--log=/something/daemon.log", which makes them run "ini_set('error_log',
$that_log);". This is generally good because it casues errors to appear in the
log.

However, it causes errors to stop appearing on stderr, which means the daemon
overseer can't capture stderr anymore and can't send it to the web console. It
thus reports failures as just an exit code instead of something more useful.

Test Plan: Ran "phd launch mercurialpull ..." on an intentionally bad repository
configuration and got full information about the issue in the web and disk logs.

Reviewers: Makinde, jungejason, btrahan, nh, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 1101