Mk/Scripts/do-users-groups.sh: Make users and groups creation fail-fast

Fail fast when pw(8) fails to create a user or a group. Especially when it is
not the last command in the pre-install script then the exit code will be 0 and
the failure will go unnoticed.

PR: 267384
Approved by: jrm (mentor)
Differential Revision: https://reviews.freebsd.org/D42719