Add basic CloudWatch alarms

[jakebolam]

Is your feature request related to a problem? Please describe.
If users start depending on the bot, they want it to work. Let's ensure we know when it's not.

Describe the solution you'd like
Cloud watch alarms on error rate increases etc.
Measuring timing would be important too, too fast and something is wrong, same as too slow.

Describe alternatives you've considered

Additional context
Something like https://github.com/ACloudGuru/serverless-plugin-aws-alerts could help, instead of pure cloud formation