Improve contract api validations errors
CHANGELOG:
### REFACTOR
- [#417](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/417) Improve error messages in contract API for tech / supplier dependencies
Edited by Pelayo García
CHANGELOG:
### REFACTOR
- [#417](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/417) Improve error messages in contract API for tech / supplier dependencies