Turn some non-fatal ERROR logs into WARNING.

Summary:
This is part my attempt to clean up the ERROR logs to be things that
we really need to pay attention to.  Fatal errors that cause a
mapreduce to fail: absolutely.  Errors that just cause some code to
retry: not so much.

Test Plan: Fingers crossed.

Reviewers: tony, alan

Reviewed By: alan

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D11638