don't allow to delete an event if someone is registered

src/model/planned_event.rs

@@ -240,11 +240,29 @@ INNER JOIN trip_details ON planned_event.trip_details_id = trip_details.id",
         .unwrap(); //Okay, as planned_event can only be created with proper DB backing
     }
 
-    pub async fn delete(&self, db: &SqlitePool) {
+    pub async fn delete(&self, db: &SqlitePool) -> Result<(), String> {
+        if Registration::all_rower(db, self.trip_details_id)
+            .await
+            .len()
+            > 0
+        {
+            return Err(
+                "Event kann nicht gelöscht werden, weil mind. 1 Ruderer angemeldet ist.".into(),
+            );
+        }
+        if Registration::all_cox(db, self.trip_details_id).await.len() > 0 {
+            return Err(
+                "Event kann nicht gelöscht werden, weil mind. 1 Steuerperson angemeldet ist."
+                    .into(),
+            );
+        }
+
         sqlx::query!("DELETE FROM planned_event WHERE id = ?", self.id)
             .execute(db)
             .await
             .unwrap(); //Okay, as PlannedEvent can only be created with proper DB backing
+
+        Ok(())
     }
 
     pub async fn get_ics_feed(db: &SqlitePool) -> String {