* Improve iam_group exception handling
Use AnsibleAWSModule for iam_group and handle BotoCoreErrors
as well as ClientErrors. Use fail_json_aws to improve error messages
* Add minimal iam_group test suite
Update some of the read-only IAM permissions (this is not sufficient
to run the test suite but it gets further than it did until it tries
to add a (non-existent) user)
* Clean up after tests
