Triggering an unscheduled job should display a help message instead of error

When a job has been added to the scheduler, but hasn't been scheduled yet (with the "schedule" action), using the "trigger" action displays an error "Error number 90012 in 5: Exception in plugin [com.xpn.xwiki.plugin.scheduler.SchedulerPlugin]: Error occured while trying to trigger job MyTestJob"

This is not easy to understand.

A message like "The job needs to be scheduled before triggering it" could at least help a user.