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

[fbouffetier]

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

[Betree]

Than you for reporting this.

The bug occurs because there's already an entry for this Wikidata ID, and the system only allow one entry per Wikipedia ID.

So we should indeed merge the two speakers (I'll take care of that). A better error message would also be nice.