I have a form where I have a MMD field named 'Department'. I want this field to contain unique values only. I have easily done this in the SharePoint list settings by making the field unique. When I try and submit the form using a department value that already exists the user receives an error which is correct behaviour however the Nintex form error is poor as it says, 'Something went wrong' instead of something useful like 'Department already exists'. I have amended the controls custom error message but this does not show...it may require some Java. Any idea's?
I'm not sure why your custom error message isn't shown. However, Aaron Labiosa created a very nice post about custom validation possibilities that includes a custom error message:
Maybe it helps to get you started with some JS.