Add a clearer error message when the attempt to (un)enrol fails.

The error message returned by jQuery only contained "Error". This commit 
replaces it by a hard-coded, more descriptive error message in French.

event/static/js/calendar.js

@@ -855,8 +855,8 @@ class EventDetails {
           event.updateNodeStyle();
         },
 
-        error: (_, message) => {
-          alert(message);
+        error: () => {
+          alert("Erreur lors de l'inscription ou de la désinscription à cette permanence.");
         }
       });
     });