[FIX] stock : prevent creating sequences with same code

Issue:
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You can create the same sequence with the same code

Steps to reproduce the error:
=============================
- Install inventory and activate storage locations
- Go to inventory/configuration/Operations Types
- Create 2 operation typs with the following values:
name :any random name , type of Operations : internal transfer,
sequence prefix : test , locations as WH/Stock
- Go to sequences and search for test
- You will have 2 duplicate sequences with the same values

Origin of the problem :
=======================
- Creating an operation type always creates atuomatically a sequences if
  the sequence_code is provided but the sequence_id isn't.

Solution:
=========
Display an error when the name already exist.

opw-3238331

closes odoo/odoo#129394

Signed-off-by: William Henrotin (whe) <whe@odoo.com>