There is not an easy way to get detailed output from an action.  The 'message' field from the schedule.* calls never contains any useful info.  The only way to get the detailed messages is to call system.listSystemEvents on each system you got from schedule.list{InProgress,Completed,Failed} and then filter it locally.

A call like schedule.getActionResults(session, actionID) which returns the detailed results of an action (e.g., installed package list, configuration file details) would be very useful.