Checkmk Werk 15980: user: return 400 status code when trying to create or edit an user specifying a non existing customer

ID: 15980 Title: user: return 400 status code when trying to create or edit an user specifying a non existing customer Component: REST API Level: 1 Class: Bug fix Version: 2.3.0b1 Prior to this werk, the user setup endpoint returned a 500 status code whenever the user tried to create or update an user specifiying a non-existing customer. This werk fixes this and returns a 400 status code instead