Monitoring and alerting for 500 responses

There should be monitoring and alert thresholds for the number of 500 responses served by CodeWorld. This would have caught a recent problem that led to a partial outage of shared programs for two weeks.

Before we can do alerting, though, need to fix #705 so I don't get paged every time a student goes crazy with compile errors.

No files, tests, or entry points are named. Start by resolving the dependency on issue #705, then inspect the project's monitoring and deployment configuration to determine how 500 responses can be counted and thresholded. Done means alerts exist for meaningful 500-response levels without paging on expected compile errors.

Written by the indexing model from the issue text.