Refactor DossiersController#update

Nested ifs are heavier but easier to understand
as related conditions are visually grouped

app/controllers/new_user/dossiers_controller.rb

@@ -141,7 +141,8 @@ module NewUser
       if errors.present?
         flash.now.alert = errors
         render :modifier
-      elsif current_user.owns?(dossier)
+      else
+        if current_user.owns?(dossier)
           if Flipflop.new_dossier_details?
             redirect_to demande_dossier_path(@dossier)
           else
@@ -151,6 +152,7 @@ module NewUser
           redirect_to users_dossiers_invite_path(@dossier.invite_for_user(current_user))
         end
       end
+    end
 
     def merci
       @dossier = current_user.dossiers.includes(:procedure).find(params[:id])