Replace _constraints with validate in ModelStorage and improve user messages.

issue2616
review371002

work.xml

@@ -64,5 +64,14 @@ this repository contains the full copyright notices and license terms. -->
             <field name="model">project.work</field>
         </record>
 
+        <record model="ir.translation" id="project_work_recursion_error">
+            <field name="name">project.work</field>
+            <field name="lang">en_US</field>
+            <field name="type">error</field>
+            <field name="src">recursion_error</field>
+            <field name="value">Recursion error: Project Work "%(rec_name)s" with parent "%(parent_rec_name)s" was configured as ancestor of itself.</field>
+            <field name="module">project_plan</field>
+            <field name="fuzzy" eval="False"/>
+        </record>
     </data>
 </tryton>