[FIX] base: no crash when setting wrong relation to m2m

Before this rev., in the ir.model.fields form view, it crashed
when the user specified an invalid model name as a relation for
fields of type many2many (the onchange crashed). This rev. displays
a warning instead.

opw 1837401

openerp/addons/base/ir/ir_model.py

@@ -376,6 +376,13 @@ class ir_model_fields(osv.osv):
     def _onchange_ttype(self):
         self.copy = (self.ttype != 'one2many')
         if self.ttype == 'many2many' and self.model_id and self.relation:
+            if self.relation not in self.env:
+                return {
+                    'warning': {
+                        'title': _('Model %s does not exist' % self.relation),
+                        'message': _('Please specify a valid model for the object relation'),
+                    }
+                }
             names = self._custom_many2many_names(self.model_id.model, self.relation)
             self.relation_table, self.column1, self.column2 = names
         else: