Patron editor: "cannot manage role" error is not clear

[PascalRepond]

Bug description:

Trying to manage a role that I am not allowed to gives a console error, but the UI only show a red bar.

Expected behavior:

Display the backend validation error in the UI in the red bar. E.g.: "Unable to manage role(s) pro_statistic_manager"

Steps to Reproduce:

1. Log in as Virgile
2. Edit a patron and try to add professionnal role "pro statistics manager"
3. See error