[FIX] hr_expense: missing exception import

In hr_expense.py, a ValidationError exception can be raised in some
cases but it's not imported from odoo.exceptions

addons/hr_expense/models/hr_expense.py

@@ -4,7 +4,7 @@
 import re
 
 from odoo import api, fields, models, _
-from odoo.exceptions import UserError
+from odoo.exceptions import UserError, ValidationError
 from odoo.tools import email_split, float_is_zero
 
 from odoo.addons import decimal_precision as dp