Don't report significant errors unless they are new or have > 4 instances/min.

Summary:
When looking for "new" errors, check all 10 minutes of each previous version's
monitoring data to catch low-frequency (fewer than 5 instances per minute)
errors. This should reduce the spam from errors that happen infrequently enough
that they may not have been reported in the corresponding previous timeslice.

Test Plan: `python test_server.py`

Reviewers: jlfwong, csilvers

Reviewed By: jlfwong, csilvers

Subscribers: jace

Differential Revision: http://phabricator.khanacademy.org/D14688