JCLOUDS-529: fix aws-ec2 cleanupIncidentalResources

- Some users get a DependencyVioloation, rather than InvalidGroup.InUse,
  when attempting to delete the security group. This caused
  cleanupIncidentalResources to propagate an exception.
- Fixes it by converting this to an IllegalStateException (in same
  way as is done for “InUse”)
- Adds tests (using MockWebServer) for happy-path and for failing
  to delete the security group with each of InUse and DependencyViolation
  responses.