Better monitoring + testing for Cost Watcher Lambda function

[hcho3]

The Cost Watcher Lambda function currently lacks any unit tests. The function is already 262 lines long.

• Monitor number of successful and unsuccessful function calls. This is done in CloudWatch.
• Keep logs of all function executions. Again, CloudWatch.
• Set up automatic e-mail alarm for any failures. I've set up a CloudWatch Alarm and put it my e-mail there.
• Set up mock testing: Does each component function behave in expected ways?
• Add more assertions to enforce invariants and pre- / post-conditions.
• Consider fault injection testing.