Error monitor: Additional fetching logic for error data (to support new UI)

Summary:
- New API endpoints to fetch all recent errors or errors by version (summary
  data only)
- Additional error information including routes, URLs, and stack traces for
  the single-error fetching API

Test Plan:
- `python test_server.py`
- Tested in conjunction with changes in webapp

Reviewers: jlfwong, csilvers

Reviewed By: csilvers

Subscribers: jace

Differential Revision: https://phabricator.khanacademy.org/D15261