Support graph nodes as default error handlers

Reported by @svensson

        {
            "id": "CommonErrorReporter",
            "task_type": "graph",
            "task_identifier": "CommonErrorReporter.json",
            "label": "CommonErrorReporter",
            "default_error_node": true
        }

This means all input nodes of CommonErrorReporter should become default error handlers in the supergraph.