The business logic may be such that you want to return a warning when saving an entity. Not an error, a warning.

Example, I'm saving the age of a resident entity. If the age is less than 50 [that is not an error], I want to warn the user that this may be an incorrect age for a resident based on my business rules. I want this business rule implemented on the server side so it is available to all clients.

When a warning is detected, the entity would still be saved but a warning message would be returned. The client side would also reflect the updated status (including all server side update to the entity) but the message would be accessible just like errors are today.

You can implement this by returning a message collection containing both errors and warnings with a severity type or two different collections, one with errors and one with warnings. Each as it's pros and cons.