Add a setting to show which entities with the same values already exist in the error message

Problem/Motivation

Thank you very much for this very helpful module.

One important thing missing is to let the user (optionally by setting, default TRUE) know, which other entity already exists with the same value.
Currently, the message only says:

Error message
The combination of fields including title, body has to be unique.

This would help users a lot to identify if the existing entity already is what they want and to compare it.

Steps to reproduce

Try creating an entity which would be a duplicate, but don't know which entity this would duplicate.

Proposed resolution

Add a setting to show the already existing entity which the new entry would be a duplicate of, e.g.

Error message
An entity "%label" with the unique combination of fields: title, body already exists. No further content with this combination is allowed.

Remaining tasks

User interface changes

API changes

Data model changes