isr_admin: Bail out of adduser if the account already exists
isr_admin: Bail out of adduser if the account already exists

- adduser(): If the account already exists, fail with an error message

- Treat a failure of /usr/sbin/useradd as a fatal error

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