Make error response more helpful

follow up to https://github.com/probot/adapter-aws-lambda-serverless/pull/59#discussion_r573401784 by @tcbyrd 

> Could this be more helpful? For example maybe point users at [docs for Cloudwatch logs](https://docs.aws.amazon.com/lambda/latest/dg/monitoring-cloudwatchlogs.html) on how to debug?

