isr_admin: Clarify an error message in prune
isr_admin: Clarify an error message in prune

- Clarify the error message produced if the user attempts to prune all
versions of a parcel

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr