Halt execution when no ECS cluster found

[rpappalax]

ExecutionFailed
{"error":"ValidationException","cause":"{\"stackTrace\": [[\"/var/task/handler.py\", 13, \"populate_missing_instances\", \"return runner.populate_missing_instances()\"], [\"/var/task/ardere/step_functions.py\", 190, \"populate_missing_instances\", \"self._validate_plan()\"], [\"/var/task/ardere/step_functions.py\", 179, \"_validate_plan\", \"raise ValidationException(\\\"Failed to validate: {}\\\".format(errors))\"]], \"errorType\": \"ValidationException\", \"errorMessage\": \"Failed to validate: {'ecs_name': ['No cluster with the provided name.']}\"}"}