better error handling in user/group creation

[jgoodall]

the user account creation was failing on centos 5.8, due to, first, the useradd binary not being in my path and then to useradd not having the command option --user-group. i tried to make the function more robust and at least print an error that the account and group need to be manually created if the groupadd and useradd binaries cannot be found.