No longer possible to get ID of failed node

[kyrias]

Previously the DagError::TaskError type contained the task name, task ID, and the node-provided error string. The new equivalent of this error type appears to be GraphError::ExecutionFailed but it only contains a formatted string.

It would be more user friendly if it still contained node_name, node_id, and error as struct members and then have the Display implementation build the string instead so that our error handling can reliably get the actual node ID that failed.