[FIX] account_voucher: allow the web client to catch the warning instead of returning the whole traceback

Actually, it was a Python ``Warning`` exception that was returned, and
not an ``openerp.exceptions.Warning``, that is an alias of ``openerp.exceptions.UserError``.

addons/account_voucher/account_voucher.py

@@ -144,7 +144,7 @@ class AccountVoucher(models.Model):
     def unlink(self):
         for voucher in self:
             if voucher.state not in ('draft', 'cancel'):
-                raise Warning(_('Cannot delete voucher(s) which are already opened or paid.'))
+                raise UserError(_('Cannot delete voucher(s) which are already opened or paid.'))
         return super(AccountVoucher, self).unlink()
 
     @api.multi