Handle existing Boefje name

This PR handles the exception (in the form) that occurs when a user tries to create/edit a Boefje with a name that already exists.

Followup of #3521

Adds an error message to the name field (lay-out of error message will be fixed in #3522)
Optimizes the code
AddBoefjeVariantView now redirects to the new plugin_id (instead of the "original" boefje id.

Closes #3566 Closes #3586

afbeelding

Please add some information for QA on how to test the newly created code.

[x] All the commits in this PR are properly PGP-signed and verified.
[x] This PR only contains functionality relevant to the issue.
[ ] I have written unit tests for the changes or fixes I made. // No new tests needed
[x] I have checked the documentation and made changes where necessary.
[x] I have performed a self-review of my code and refactored it to the best of my abilities.

[ ] Tickets have been created for newly discovered issues.
[ ] For any non-trivial functionality, I have added integration and/or end-to-end tests.
[ ] I have informed others of any required .env changes files if required and changed the .env-dist accordingly.
[ ] I have included comments in the code to elaborate on what is not self-evident from the code itself, including references to issues and discussions online, or implicit behavior of an interface.

Checklist for code reviewers:

Copy-paste the checklist from the docs/source/templates folder into your comment.

Copy-paste the checklist from the docs/source/templates folder into your comment.