[IMP] hr_holidays: show conflicting time off in validation error message

Description of the issue/feature this PR addresses:
In hr_holidays, there is no information about the conflicting time off when a second time off is scheduled at the same time.
There is a validation error saying "You can not set 2 time off that overlaps on the same day for the same employee".
The validation error should display the conflicting time off

Current behavior before PR:
The validation error displays the following:
"You can not set 2 time off that overlaps on the same day for the same employee"

Desired behavior after PR is merged:
The validation error will display the following:
"You can not set 2 time off that overlaps on the same day for the same employee.
Existing time off: Mitchell Admin / Trip with Family : 24.00 hours / from 06/09/2021 to 08/09/2021 / To Approve"

task-2641682

closes odoo/odoo#76452

Related: odoo/enterprise#22490
Signed-off-by: Kevin Baptiste <kba@odoo.com>