Improve error message when there's a duplicate for a wikidata speaker

**When try to save update on character, error message for platform**

****
***

Then, when click on save, error message appears => ****

** Character https://captainfact.io/s/Isabelle-Saporta need to be updated or merged with Character https://captainfact.io/s/6650**

**System:**

- Windows 10
- Firefox 73.0.1

Start with the character save/update flow described in the issue and reproduce the duplicate Wikidata-speaker case using the two linked character pages. Done means the resulting error clearly identifies the duplicate relationship and tells the user which characters need updating or merging; no file or test is named in the report.

Written by the indexing model from the issue text.