User friendly Error for unique field - MongoDB with Golang

Hey ! I am using MongoDB with Go and stuck in a problem. I build a user authentication API in registration I want user to have unique Email. I successfully create an index for email but now I want user friendly error. My API is working fine and just want to show some user friendly error to user like when he/she tried to duplicate the data the pop up should appear in browser containing user friendly message. For example “This email is already registered”

Can anyone help me with that !
Any help would be greatly appreciated.