Add debug output

PagerDuty#16)

* add try/except around slack channel updates to handle keyerror when the bot is not allowed or channel is archived

* move error handling to initial get channel call to catch errors where the channel id cannot be found

* change payload[channel] to just channel cause its clear and less letters

https://aws.amazon.com/blogs/compute/upcoming-changes-to-the-python-sdk-in-aws-lambda/

Vendored requests is being removed from the Python 3.6 runtime on Jan
31. The Python 3.8 runtime is unaffected.

Other ways to address this deprecation would be to replace requests
usage with urllib3 usage, or to add a dependency on a lambda layer which
provides requests.

Pin lambda layer so requests keeps working