(wrote by LSE)

Before this commit:
 If an error happened when trying to synchronise a PoS order nothing is saved on the backend to inform the user regarding the error.
 Note: Odoo logs would contain the information, but it is out of reach for certain clients (on odoo online for instance).
 
 In theory, we can't lose any information as, if the sync process raise an exception, the order is still on the PoS browser cache that will  then try to be resync when another order sync happen.
 But, in practice, the support received some cases of "missing PoS  orders". Which can happen as we fully rely on the client browser cache that can be cleared or use another computer/browser/session.

After this commit:
 If an order can not be processed in the backend:
  - the PoS order data is saved in the PoS session attachments
  - a scheduled activity is created in the PoS session
 
 As an un-synced keep being tried to be sync (and will likely fail each time), we compare it with the already attached one to avoid having the content repeated multiple times.
 If the order was modified in between, a new attachment with the same name is created.
 
 Note: draft orders that will fail to validate are NOT stored
 
 The attachment and activity are automatically removed when the order of same reference is validated

![image](https://github.com/odoo/odoo/assets/60775325/c3a503f3-2528-44d3-b720-c82776f40238)

opw-3650239

Forward-Port-Of: odoo/odoo#167269
Forward-Port-Of: odoo/odoo#147130