I reproduced the error by trying to move an existing device around using a non-admin account. The net join message is

"Failed to join domain: failed to precreate account in ou ou=Chromad Dev Devices,ou=Chromad Dev,dc=<DEVICE_REALM>,dc=<DEVICE_REALM>: Constraint violation"

This maps to ERROR_OU_ACCESS_DENIED, which maps to 

"Can't join the device to the domain. Check your account to make sure you have privileges to add devices."

I'll map Constraint violation to a different error.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/system_api/+/78d910462f762d6abfdfc6d3d3ada185a47be2c1

commit 78d910462f762d6abfdfc6d3d3ada185a47be2c1
Author: Lutz Justen <ljusten@chromium.org>
Date: Thu Jul 19 13:12:26 2018

authpolicy: Add ERROR_OU_CONSTRAINT_VIOLATION

The error is use to improve the error message in Chrome when an
unprivileged user tries to join a device with a machine name that
already exists elsewhere.

BUG=chromium:864997
TEST=Tested on device

Change-Id: I6c407cd04fb60712b00f2653d12f204c4758982b
Reviewed-on: https://chromium-review.googlesource.com/1141955
Commit-Ready: Lutz Justen <ljusten@chromium.org>
Tested-by: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Roman Sorokin <rsorokin@chromium.org>

[modify] https://crrev.com/78d910462f762d6abfdfc6d3d3ada185a47be2c1/dbus/authpolicy/active_directory_info.proto

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform2/+/977874a71cd3c6264ea79e2b18dde689c92a27d5

commit 977874a71cd3c6264ea79e2b18dde689c92a27d5
Author: Lutz Justen <ljusten@chromium.org>
Date: Thu Jul 19 13:12:27 2018

authpolicy: Return separate error for constraint violation

Returns a separate error code for a constraint violation while setting
OU for domain join. This happens when an unprivileged user tries to join
a device with a machine name that already exists elsewhere. The error is
use to improve the error message in Chrome.

CQ-DEPEND=CL:1141955

BUG=chromium:864997
TEST=Tested on device

Change-Id: Ibece7e805aaa61feeab398a491203ddd9d1e4fbd
Reviewed-on: https://chromium-review.googlesource.com/1141956
Commit-Ready: Lutz Justen <ljusten@chromium.org>
Tested-by: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Roman Sorokin <rsorokin@chromium.org>

[modify] https://crrev.com/977874a71cd3c6264ea79e2b18dde689c92a27d5/authpolicy/samba_interface.cc

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/autotest-private/+/667096d5b860ff3b0bc2e89d676982ab6e0b9944

commit 667096d5b860ff3b0bc2e89d676982ab6e0b9944
Author: Lutz Justen <ljusten@chromium.org>
Date: Tue Jul 31 14:15:46 2018