feat: show error message for invitation accept

client/src/pages/learningMentor/InvitationAcceptPage.vue

@@ -7,7 +7,15 @@ const props = defineProps<{
   invitationId: string;
 }>();
 
-const { data, error } = useCSRFFetch(`/api/mentor/${props.courseId}/invitations/accept`)
+const { data, error } = useCSRFFetch(
+  `/api/mentor/${props.courseId}/invitations/accept`,
+  {
+    onFetchError(ctx) {
+      ctx.error = ctx.data;
+      return ctx;
+    },
+  }
+)
   .post({
     invitation_id: props.invitationId,
   })
@@ -31,6 +39,9 @@ const { data, error } = useCSRFFetch(`/api/mentor/${props.courseId}/invitations/
             <div>
               <a class="underline" href="mailto:help@vbv.ch">help@vbv.ch</a>
             </div>
+            <div v-if="error.message" class="my-4">
+              {{ $t("a.Fehlermeldung") }}: {{ error.message }}
+            </div>
           </template>
           <template v-else>
             <i18next