Don't run error rate analysis for previous versions we don't have data for.

Summary:
When we receive monitoring log data for a version and minute, set a flag
that tracks that we've seen it so when we do the analysis we can filter out
versions for which we haven't gotten data (which will happen for the first
few deploys after monitoring is enabled but also for versions where the
deployer manually skips monitoring)

Test Plan: `python test_server.py`

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: jace

Differential Revision: http://phabricator.khanacademy.org/D14603