Bump client_max_body_size in nginx config for error-monitor-db.

Summary:
Saw in nginx logs on prod-rpc that requests have been failing due to a
large POST body. Hopefully 4MB will be enough.

Test Plan: Crossed fingers.

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: jace

Differential Revision: http://phabricator.khanacademy.org/D14694