Unit tests for failures

[sandrinodm]

Currently cron jobs run in the same container that are also serving rules and custom database connections. If an unhandled exception happens here, or the callback is not called for any reason, this might cause the container to be quarantined (impacting rules and database connections).

It would be good to have some unit tests that test the actual webtask and trigger some errors:

• Error from API v2
• Error when initializing Mailchimp
• Error when interacting with mailchimp

I noticed the harness folder (which are more like integration tests), but ideally here we would just have some unit tests with mocks.