Improve error logging "Instance is not unique"

Could you make the error “Instance is not unique” clearer? I suggest adding the corresponding Index that cause the unique error and the argument field. E.g “Instance is not unique with Index 'Foo 'at the argument field ‘bar’”

Every time I got into this error, I have to trace which argument field is invalid with which index, which takes a lot of time when the system is complicated.

Thanks in advance.