I'm going to be working on a Map/Reduce script soon and I would like to use the summarize section to save performance information to a custom record (e.g how many of the input lines processed successfully). I remember seeing stuff like

summary.mapSummary.errors.iterator().each

, can I add my own details to the error information? Do things only get added to the error section if a script error occurs or could I, for example, catch the initial error, do some stuff and then throw an error myself?