This is a quick and short article which talks about displaying ADF BC REST resource's validation message on the JET UI.
The inital step to is to create a validator method on one of the desired attributes of the ADF entity object. For my use case, I have selected Salary attribute on Employee EO.
While calling create or save on your collection data, the error callback has 3 parameters. The first parameter contains all the details of the error thrown. The responseText attribute shows the exact error text thrown from the REST end-point.
On encountering any error, the details are popped onto the UI. The messages are displayed via oj-messages component.