Add ir.message and use custom exceptions

issue3672

exceptions.py

@@ -0,0 +1,12 @@
+# This file is part of Tryton.  The COPYRIGHT file at the top level of
+# this repository contains the full copyright notices and license terms.
+
+from trytond.model.exceptions import ValidationError
+
+
+class AccountValidationError(ValidationError):
+    pass
+
+
+class RootValidationError(ValidationError):
+    pass