Don't log Conduit 404s as errors

Summary: Fixes T5695. A Conduit "method does not exist" exception is somewhat expected... there is no need to phlog the exception.

Test Plan: Called a non-existent Conduit method. Saw no exceptions in the error logs.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5695

Differential Revision: https://secure.phabricator.com/D10042