Use message for unique constraint on model data

trytond/trytond/ir/message.xml

@@ -429,5 +429,8 @@ this repository contains the full copyright notices and license terms. -->
         <record model="ir.message" id="msg_cache_name_unique">
             <field name="text">The name of cache must be unique.</field>
         </record>
+        <record model="ir.message" id="msg_model_data_xml_id_model_unique">
+            <field name="text">The XML ID must be unique per model.</field>
+        </record>
     </data>
 </tryton>

trytond/trytond/ir/model.py

@@ -1310,7 +1310,7 @@ class ModelData(
         cls._sql_constraints = [
             ('fs_id_module_model_uniq',
                 Unique(table, table.fs_id, table.module, table.model),
-                'The triple (fs_id, module, model) must be unique!'),
+                'ir.msg_model_data_xml_id_model_unique'),
             ]
         cls._sql_indexes.update({
                 Index(