eks__enum crashes when failing cluster enumeration

[davidkutz-marks]

When used with keys that do not permit EKS::ListClusters, Pacu prints the following error and crashes:

"[eks_enum] Error: An error occurred (UnrecognizedClientException) when calling the ListClusters operation: The security token included in the request is invalid"

I am adding error handling for this and will issue a PR shortly.