Comment 2 for bug 1492327

just touching /var/lib/extrausers/subuid and /var/lib/extrausers/subgid seems to be enough to make this particulare error go away, but it then fails in chfn:

(RaspberryPi2)ubuntu@localhost:~$ sudo adduser --extrausers testuser
Adding user `testuser' ...
Adding new group `testuser' (1001) ...
ENTER EXTRAUSERS_GROUP_FILEEXIT EXTRAUSERS_GROUP_FILEENTER EXTRAUSERS_SHADOWGROUP_FILEEXIT EXTRAUSERS_SHADOWGROUP_FILEAdding new user `testuser' (1001) with group `testuser' ...
Creating home directory `/home/testuser' ...
Copying files from `/etc/skel' ...
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
/usr/bin/chfn: unrecognized option '--extrausers'
Usage: chfn [options] [LOGIN]

Options:
  -f, --full-name FULL_NAME change user's full name
  -h, --home-phone HOME_PHONE change user's home phone number
  -o, --other OTHER_INFO change user's other GECOS information
  -r, --room ROOM_NUMBER change user's room number
  -R, --root CHROOT_DIR directory to chroot into
  -u, --help display this help message and exit
  -w, --work-phone WORK_PHONE change user's office phone number

adduser: `/usr/bin/chfn --extrausers testuser' returned error code 2. Exiting.
(RaspberryPi2)ubuntu@localhost:~$